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<body class="body">

<!-- INDEX s7doc:s7 scheme -->


<div class="topheader" id="s7doc">s7
</div>


<p>s7 is a Scheme implementation intended as an extension language
for other applications, primarily Snd, Radium, and Common Music.  It exists as just two files, s7.c and
s7.h, that want only to disappear into someone else's source tree.  There are no libraries,
no run-time init files, and no configuration scripts.
It can be built as a stand-alone
interpreter (see <a href="#repl">below</a>).  s7test.scm is a regression test for s7.
A tarball is available: <a href="ftp://ccrma-ftp.stanford.edu/pub/Lisp/s7.tar.gz">s7 tarball</a>.
</p>

<p>
s7 is the default extension language of Snd and sndlib (<a href="http://ccrma.stanford.edu/software/snd/index.html">snd</a>),
Rick Taube's Common Music (commonmusic at sourceforge), and Kjetil Matheussen's Radium music editor.  
There are X, Motif, Gtk, and openGL bindings
in libxm in the Snd tarball, or at ftp://ccrma-ftp.stanford.edu/pub/Lisp/libxm.tar.gz.
If you're running s7 in a context
that has getenv, file-exists?, and system, you can use s7-slib-init.scm
to gain easy access to slib.  This init file is named "s7.init" in the slib distribution.
</p>

<p>Although it is a descendant of tinyScheme, s7 is closest as a Scheme dialect to Guile 1.8.
I believe it is compatible with <a href="#s7vsr5rs">r5rs</a> and <a href="#r7rs">r7rs</a>:  you can just ignore all the additions discussed in this file.
It has continuations, 
ratios, complex numbers,
macros, keywords, hash-tables, 
multiprecision arithmetic,
generalized set!, unicode, and so on.
It does not have syntax-rules or any of
its friends, and it does not think there is any such thing
as an inexact integer.  
</p>

<p>This file assumes you know about Scheme and all its problems,
and want a quick tour of where s7 is different.  (Well, it was quick once upon a time).
The main difference: if it's in s7, it's a first-class citizen of s7, and that includes
macros, environments, and syntactic forms.
</p>

<br>
<blockquote>
<div class="indented">
<p>I originally used a small font for scholia, but now I have to squint
to read that tiny text, so less-than-vital commentaries are shown in the normal font, but 
indented and on a sort of brownish background.
</p>
</div>
</blockquote>
<br>

<!--
<div class="orange">
<p>Danger!</p>
<p>Men Working</p>
<p>in Trees</p>
</div>
-->

<ul>
  <li><a href="#multiprecision">arbitrary precision arithmetic</a>
  <li><a href="#math">math functions</a>
  <li><a href="#define*">define*, named let*</a>
  <li><a href="#macros">define-macro</a>
  <li><a href="#pws">procedure-setter</a>
  <li><a href="#generalizedset">generalized set!</a>
  <li><a href="#multidimensionalvectors">multidimensional vectors</a>
  <li><a href="#hashtables">hash tables</a>
  <li><a href="#environments">environments</a>
  <li><a href="#multiplevalues">multiple-values</a>
  <li><a href="#callwithexit1">call-with-exit</a>
  <li><a href="#format1">format</a>
  <li><a href="#hooks">hooks</a>
  <li><a href="#procedureinfo">procedure info</a>
  <li><a href="#evalstring">eval</a>
  <li><a href="#IO">IO and other OS functions</a>
  <li><a href="#errors">errors</a>
  <li><a href="#autoload">autoload</a>
  <li><a href="#constants">define-constant, symbol-access</a>
  <li><a href="#miscellanea">marvels and curiousities:</a> 

  <ul>
    <li><a href="#loadpath">*load-path*</a>, <a href="#featureslist">*features*</a>, <a href="#sharpreaders">*#readers*</a>,
    <li><a href="#makelist">make-list</a>, <a href="#charposition">char-position</a>, <a href="#keywords">keywords</a>
    <li><a href="#symboltable">symbol-table</a>, <a href="#s7help">help</a>, <a href="#s7gc">gc</a>, <a href="#morallyequalp">morally-equal?</a>
    <li><a href="#expansion">define-expansion</a>, <a href="#s7env">*s7*</a>, <a href="#s7vsr5rs">r5rs</a>, <a href="#r7rs">r7rs</a>, 
    <li><a href="#profiling">profiling</a>, <a href="#circle">circular lists</a>, <a href="#legolambda">legolambda</a>, etc...
    </ul>

  <li class="li_header"><a href="#FFIexamples">FFI examples</a>
  <ul>
      <li><a href="#repl">read-eval-print loop (and emacs)</a>
      <li><a href="#defun">define a function with arguments and a returned value, and define a variable </a>
      <li><a href="#defvar">call a Scheme function from C, and get/set Scheme variable values in C</a>
      <li><a href="#juce">C++ and Juce</a>
      <li><a href="#sndlib">load sndlib using the Xen functions and macros</a>
      <li><a href="#pwstype">add a new Scheme type and a procedure with a setter</a>
      <li><a href="#functionportexample">redirect display output to a C procedure</a>
      <li><a href="#extendop">extend a built-in operator ("+" in this case)</a>
      <li><a href="#definestar1">C-side define* (s7_define_function_star)</a>
      <li><a href="#definemacro1">C-side define-macro (s7_define_macro)</a>
      <li><a href="#definegeneric">define a generic function in C</a>
      <li><a href="#signal">signal handling (C-C to break out of an infinite loop)</a>
      <li><a href="#vector">direct multidimensional vector element access</a>
      <li><a href="#notify">notification in C that a Scheme variable has been set!</a>
      <li><a href="#namespace">Load C defined stuff into a separate namespace</a>
      <li><a href="#Cerrors">Error handling in C</a>
      <li><a href="#testhook">Hooks in C and Scheme</a>
      <li><a href="#dload">Load a C module dynamically</a>
      <li><a href="#gmpex">gmp and friends</a>
      <li><a href="#glistener">glistener.c</a>
      <li><a href="#gdb">gdb</a>
  </ul>

  <li class="li_header"><a href="#s7examples">s7 examples</a>
  <ul>
      <li><a href="#cload">cload.scm</a>
      <ul>
           <li><a href="#libc">libc</a>
           <li><a href="#libgsl">libgsl</a>
           <li><a href="#libgdbm">libgdbm</a>
      </ul>
      <li><a href="#schemerepl">repl.scm</a>
      <li><a href="#lint">lint.scm</a>
  </ul>
</ul>


<div class="header" id="multiprecision"><h4>multiprecision arithmetic</h4></div>

<p>All numeric types, integers, ratios, reals, and complex numbers are supported.
The basic integer and real
types are defined in s7.h, defaulting to long long int and double.  
A ratio consists of two integers, a complex number two reals.
pi is predefined, as are
most-positive-fixnum and most-negative-fixnum.
s7 can be built with multiprecision support 
for all types,  using the gmp, mpfr, and mpc libraries (set WITH_GMP to 1 in s7.c).  
If multiprecision arithmetic is
enabled, the following functions are included: bignum, and bignum?, and the variable (*s7* 'bignum-precision).
(*s7* 'bignum-precision) defaults to 128; it sets the number of bits each float takes.
pi automatically reflects the current (*s7* 'bignum-precision):
</p>

<pre class="indented">
&gt; pi
<em class="gray">3.141592653589793238462643383279502884195E0</em>
&gt; (*s7* 'bignum-precision)
<em class="gray">128</em>
&gt; (set! (*s7* 'bignum-precision) 256)
<em class="gray">256</em>
&gt; pi
<em class="gray">3.141592653589793238462643383279502884197169399375105820974944592307816406286198E0</em>
</pre>

<p>
<em class=def id="bignump">bignum?</em> returns #t if its argument is a big number of some type; I use "bignum" 
for any big number, not just integers.  To create a big number,
either include enough digits to overflow the default types, or use the <em class=def id="bignum">bignum</em> function.
Its argument is a string representing the desired number:
</p>

<pre class="indented">
&gt; (bignum "123456789123456789")
<em class="gray">123456789123456789</em>
&gt; (bignum "1.123123123123123123123123123")
<em class="gray">1.12312312312312312312312312300000000009E0</em>
</pre>


<blockquote> 
<div class="indented">

<p>In the non-gmp case, if s7 is built using doubles (s7_double in s7.h), the float "epsilon" is
around (expt 2 -53), or about 1e-16.  In the gmp case, it is around (expt 2 (- (*s7* 'bignum-precision))).
So in the default case (precision = 128), using gmp:
</p>

<pre class="indented">
&gt; (= 1.0 (+ 1.0 (expt 2.0 -128)))
<em class="gray">#t</em>
&gt; (= 1.0 (+ 1.0 (expt 2.0 -127)))
<em class="gray">#f</em>
</pre>

<p>and in the non-gmp case:
</p>

<pre class="indented">
&gt; (= 1.0 (+ 1.0 (expt 2 -53)))
<em class="gray">#t</em>
&gt; (= 1.0 (+ 1.0 (expt 2 -52)))
<em class="gray">#f</em>
</pre>

<p>In the gmp case, integers and ratios are limited only by the size of memory,
but reals are limited by (*s7* 'bignum-precision).  This means, for example, that
</p>

<pre class="indented">
&gt; (floor 1e56) ; (*s7* 'bignum-precision) is 128
<em class="gray">99999999999999999999999999999999999999927942405962072064</em>
&gt; (set! (*s7* 'bignum-precision) 256)
<em class="gray">256</em>
&gt; (floor 1e56)
<em class="gray">100000000000000000000000000000000000000000000000000000000</em>
</pre>

<p>The non-gmp case is similar, but it's easy to find the edge cases:
</p>

<pre class="indented">
&gt; (floor (+ 0.9999999995 (expt 2.0 23)))
<em class="gray">8388609</em>
</pre>
</div> 
</blockquote>


<div class="header" id="math"><h4>math functions</h4></div>


<p>
s7 includes:
</p>

<ul>
<li>sinh, cosh, tanh, asinh, acosh, atanh
<li>logior, logxor, logand, lognot, logbit?, ash, integer-length, integer-decode-float
<li>random
<li>nan?, infinite?
</ul>

<p>
The random function can take any numeric argument, including 0.
The following constants are predefined: pi, most-positive-fixnum, most-negative-fixnum.
Other math-related differences between s7 and r5rs:
</p>

<ul>
<li>rational? and exact mean integer or ratio (i.e. not floating point), inexact means not exact.
<li>floor, ceiling, truncate, and round return (exact) integer results.
<li>"#" does not stand for an unknown digit.
<li>the "@" complex number notation is not supported ("@" is an exponent marker in s7).
<li>"+i" is not considered a number; include the real part.
<li>modulo, remainder, and quotient take integer, ratio, or real arguments.
<li>lcm and gcd can take integer or ratio arguments.
<li>log takes an optional second argument, the base.
<li>'.' and an exponent can occur in a number in any base.
<li>rationalize returns a ratio! 
<li>case is significant in numbers, as elsewhere: #b0 is 0, but #B0 is an error.
</ul>

<pre class="indented">
&gt; (exact? 1.0)
<em class="gray">#f</em>
&gt; (rational? 1.5)
<em class="gray">#f</em>
&gt; (floor 1.4)
<em class="gray">1</em>
&gt; (remainder 2.4 1)
<em class="gray">0.4</em>
&gt; (modulo 1.4 1.0)
<em class="gray">0.4</em>
&gt; (lcm 3/4 1/6)
<em class="gray">3/2</em>
&gt; (log 8 2)
<em class="gray">3</em>
&gt; (number-&gt;string 0.5 2)
<em class="gray">"0.1"</em>
&gt; (string-&gt;number "0.1" 2)
<em class="gray">0.5</em>
&gt; (rationalize 1.5)
<em class="gray">3/2</em>
&gt; (complex 1/2 0)
<em class="gray">1/2</em>
&gt; (logbit? 6 1) ; argument order, (logbit? int index), follows gmp, not CL
<em class="gray">#t</em>
</pre>

<p>See <a href="#libgsl">cload and libgsl.scm</a> for easy access to GSL,
and similarly libm.scm for the C math library.
</p>

<blockquote>
<div class="indented">

<p>The exponent itself is always in base 10; this follows gmp usage.
Scheme normally uses "@" for its useless polar notation, but that
means <code>(string-&gt;number "1e1" 16)</code> is ambiguous &mdash; is the "e" a digit or an exponent marker?
In s7, "@" is an exponent marker.
</p>

<pre class="indented">
&gt; (string-&gt;number "1e9" 2)  ; (expt 2 9)
<em class="gray">512.0</em>
&gt; (string-&gt;number "1e1" 12) ; "e" is not a digit in base 12
<em class="gray">#f</em>
&gt; (string-&gt;number "1e1" 16) ; (+ (* 1 16 16) (* 14 16) 1)
<em class="gray">481</em>
&gt; (string-&gt;number "1.2e1" 3); (* 3 (+ 1 2/3))
<em class="gray">5.0</em>
</pre>
</div>


<div class="indented">

<p>Should s7 predefine the numbers +inf.0, -inf.0, and nan.0?  It doesn't currently, but you can
get them via log or 1/0 (see <a href="#r7rs">below</a>).
But what is <code>(/ 1.0 0.0)</code>?  s7 gives a "division by zero" error here, and also in <code>(/ 1 0)</code>.
Guile returns +inf.0 in the first case, which seems reasonable, but a "numerical overflow" error in the second.
Slightly weirder is <code>(expt 0.0 0+i)</code>.  Currently s7 returns 0.0, Guile returns +nan.0+nan.0i,
Clisp and sbcl throw an error.  Everybody agrees that <code>(expt 0 0)</code> is 1, and Guile thinks
that <code>(expt 0.0 0.0)</code> is 1.0.  But <code>(expt 0 0.0)</code> and <code>(expt 0.0 0)</code> return different
results in Guile (1 and 1.0), both are 0.0 in s7, the first is an error in Clisp, but the second returns 1,
and so on &mdash; what a mess!  This mess was made a lot worse than it needs to be when the IEEE decreed that
0.0 equals -0.0, so we can't tell them apart, but that they produce different results in nearly every use!  
</p>

<pre class="indented">
scheme@(guile-user)&gt; (= -0.0 0.0)
<em class="gray">#t</em>
scheme@(guile-user)&gt; (negative? -0.0)
<em class="gray">#f</em>
scheme@(guile-user)&gt; (= (/ 1.0 0.0) (/ 1.0 -0.0))
<em class="gray">#f</em>
scheme@(guile-user)&gt;  (&lt; (/ 1.0 -0.0) -1e100 1e100 (/ 1.0 0.0))
<em class="gray">#t</em>
</pre>

<p>
How can they be equal? In s7, the sign
of -0.0 is ignored, and they really are equal.
One other oddity: two floats can satisfy eq? and yet not be eqv?:
<code>(eq? nan.0 nan.0)</code> might be #t (it is unspecified), but <code>(eqv? nan.0 nan.0)</code> is #f.
The same problem afflicts memq and assq.  
</p>
</div>


<div class="indented">

<p>The <em class=def id="random">random</em> function takes a range and an optional state, and returns a number 
between zero and the range, of the same type as the range.  It is perfectly reasonable
to use a range of 0, in which case random returns 0.
<em class=def id="randomstate">random-state</em> creates a new random state from a seed.  If no state is passed,
random uses some default state initialized from the current time.  <em class=def id="randomstatep">random-state?</em> returns #t if passed a random state object.
s7 ought to have a function named random? that always returns #t.
</p>

<pre class="indented">
&gt; (random 0)
<em class="gray">0</em>
&gt; (random 1.0)
<em class="gray">0.86331198514245</em>
&gt; (random 3/4)
<em class="gray">654/1129</em>
&gt; (random 1+i)
<em class="gray">0.86300308872748+0.83601002730848i</em>
&gt; (random -1.0)
<em class="gray">-0.037691127513267</em>
&gt; (define r0 (random-state 1234))
<em class="gray">r0</em>
&gt; (random 100 r0)
<em class="gray">94</em>
&gt; (random 100 r0)
<em class="gray">19</em>
&gt; (define r1 (random-state 1234))
<em class="gray">r1</em>
&gt; (random 100 r1)
<em class="gray">94</em>
&gt; (random 100 r1)
<em class="gray">19</em>
</pre>

<p>copy the random-state to save a spot in a random number sequence, or save the random-state as a list via
random-state-&gt;list, then to restart from that point, apply random-state to that list.
</p>
</div>


<div class="indented">

<p>I can't find the right tone for this section; this is the 400-th revision; I wish I were a better writer!
</p>

<p>In some Schemes,
"rational" means "could possibly be
expressed equally well as a ratio: floats are approximations".  In s7 it's: "is actually expressed (at the scheme level) as a ratio (or an integer of course)";
otherwise "rational?" is the same as "real?":
</p>

<pre class="indented">
(not-s7)&gt; (rational? (sqrt 2))
<em class="gray">#t</em>
</pre>

<p>That 1.0 is represented at the IEEE-float level as a sort of
ratio does not mean it has to be a scheme ratio; the two notions are independent.
</p>

<p>But that confusion is trivial compared to the completely nutty "inexact integer".
As I understand it, "inexact" originally meant "floating point", and "exact" meant integer or ratio of integers.
But words have a life of their own.
0.0 somehow became an "inexact" integer (although it can be represented exactly in floating
point).  
+inf.0 must be an integer &mdash;
its fractional part is explicitly zero!  But +nan.0... 
And then there's:
</p>

<pre class="indented">
(not-s7)&gt; (integer? 9007199254740993.1)
<em class="gray">#t</em>
</pre>

<p>
When does this matter?  I often need to index into a vector, but the index is a float (a "real" in Scheme-speak: its
fractional part can be non-zero).
In one Scheme:
</p>

<pre class="indented">
(not-s7)&gt; (vector-ref #(0) (floor 0.1))
<em class="gray">ERROR: Wrong type (expecting exact integer): 0.0   </em>; [why?  "it's probably a programmer mistake"!]
</pre>

<p>Not to worry, I'll use inexact-&gt;exact:
</p>

<pre class="indented">
(not-s7)&gt; (inexact-&gt;exact 0.1)
<em class="gray">3602879701896397/36028797018963968                  </em>; [why? "floats are ratios"!]
</pre>

<p>So I end up using the verbose <code>(floor (inexact-&gt;exact ...))</code> everywhere, and even
then I have no guarantee that I'll get a legal vector index. 
I have never seen any use made of the exact/inexact distinction &mdash; just
wild flailing to try get around it.
I think the whole idea is confused and useless, and leads
to verbose and buggy code.  
If we discard it,
we can maintain backwards compatibility via:
</p>

<pre class="indented">
(define exact? rational?)
(define (inexact? x) (not (rational? x)))
(define inexact-&gt;exact rationalize) ; or floor
(define (exact-&gt;inexact x) (* x 1.0))
</pre>

<p>#i and #e are also useless because you can
have any number after, for example, #b:
</p>

<pre class="indented">
&gt; #b1.1
<em class="gray">1.5</em>
&gt; #b1e2
<em class="gray">4.0</em>
&gt; #o17.5+i
<em class="gray">15.625+1i</em>
</pre>

<p>Speaking of #b and friends, what should <code>(string-&gt;number "#xffff" 2)</code> return?
</p>
</div>


<details>
<summary class="indented">more examples</summary>
<div class="indented">

<pre>
(define (log-n-of n . ints)     ; return the bits on in exactly n of ints
  (let ((len (length ints)))
    (cond ((= len 0) (if (= n 0) -1 0))
	  ((= n 0)   (lognot (apply logior ints)))
	  ((= n len) (apply logand ints))
	  ((&gt; n len) 0)
	  (#t 
	   (do ((1s 0)
		(prev ints)
		(i 0 (+ i 1)))
	       ((= i len) 1s)
	     (let ((cur (ints i)))
	       (if (= i 0)
		   (set! 1s (logior 1s (logand cur (apply log-n-of (- n 1) (cdr ints)))))
		   (let ((mid (cdr prev)))
		     (set! (cdr prev) (if (= i (- len 1)) () (cdr mid)))
		     (set! 1s (logior 1s (logand cur (apply log-n-of (- n 1) ints))))
		     (set! (cdr prev) mid)
		     (set! prev mid)))))))))
</pre>
</div>
</details>

</blockquote>


<div class="header" id="define*"><h4>define*, lambda*</h4></div>


<p><em class=def id="definestar">define*</em> and
 <em class=def id="lambdastar">lambda*</em>
are extensions of define and lambda that make it easier
to deal with optional, keyword, and rest arguments.  
The syntax is very simple: every argument to define* has a default value
and is automatically available as a keyword argument.  The default value
is either #f if unspecified, or given in a list whose first member is
the argument name.
The last argument
can be preceded by :rest or a dot to indicate that all other trailing arguments
should be packaged as a list under that argument's name.  A trailing or rest
argument's default value is ().
</p>

<pre class="indented">
(<em class=red>define*</em> (hi a (b 32) (c "hi")) (list a b c))
</pre>

<p>Here the argument "a" defaults to #f, "b" to 32, etc.
When the function is called, 
the argument names are set from the values passed the function,
then any unset arguments are bound to their default values, evaluated in left-to-right order.
As the current argument list is scanned,  any name that occurs as a keyword, :arg for example where the parameter name is arg,
sets that argument's new value.  Otherwise, as values occur, they
are plugged into the actual argument list based on their position, counting a keyword/value pair as one argument.
This is called an optional-key list in CLM.  So, taking the function
above as an example:
</p>

<pre class="indented">
&gt; (hi 1) 
<em class="gray">(1 32 "hi")</em>
&gt; (hi :b 2 :a 3) 
<em class="gray">(3 2 "hi")</em>
&gt; (hi 3 2 1) 
<em class="gray">(3 2 1)</em>
</pre>

<p>See s7test.scm for many examples.  (s7's define* is very close to srfi-89's define*).
</p>


<blockquote>

<div class="indented">
<p>The combination of optional and keyword arguments is viewed with disfavor in the Lisp
community, but the problem is in CL's implementation of the idea, not the idea itself.
I've used the s7 style since around 1976, and have never found it confusing.  The mistake 
in CL is to require the optional arguments if a keyword argument occurs, and to consider them as distinct from the
keyword arguments.  So everyone forgets and puts a keyword where CL expects a required-optional
argument.  CL then does something ridiculous, and the programmer stomps around shouting about keywords, but the fault lies with CL.
If s7's way is considered too loose, one way to tighten it might be to insist that once a keyword
is used, only keyword argument pairs can follow.  
</p>
</div>


<div class="indented">
<p>A natural companion of lambda* is named let*.  In named let, the implicit function's
arguments have initial values, but thereafter, each call requires the full set of arguments.
Why not treat the initial values as default values? 
</p>

<pre class="indented">
&gt; (let* func ((i 1) (j 2)) 
    (+ i j (if (&gt; i 0) (func (- i 1)) 0)))
<em class="gray">5</em>
&gt; (letrec ((func (lambda* ((i 1) (j 2)) 
                   (+ i j (if (&gt; i 0) (func (- i 1)) 0)))))
    (func))
<em class="gray">5</em>
</pre>

<p>This is consistent with the lambda* arguments because their defaults are
already set in left-to-right order, and as each parameter is set to its default value,
the binding is added to the default value expression environment (just as if it were a let*).
The let* name itself (the implicit function) is not defined until after the bindings
have been evaluated (as in named let).
</p>

<p>In CL, keyword default values are handled in the same way:
</p>

<pre class="indented">
&gt; (defun foo (&amp;key (a 0) (b (+ a 4)) (c (+ a 7))) (list a b c)) 
<em class="gray">FOO </em>
&gt; (foo :b 2 :a 60) 
<em class="gray">(60 2 67) </em>
</pre>

<p>In s7, we'd use:
</p>

<pre class="indented">
(define* (foo (a 0) (b (+ a 4)) (c (+ a 7))) (list a b c))
</pre>
</div>


<div class="indented">

<p>To try to catch what I believe are usually mistakes, I added two
error checks.  One is triggered if you set the same parameter twice
in the same call, and the other if an unknown keyword is encountered
in the key position.  These problems arise in a case such as
</p>

<pre class="indented">
(define* (f (a 1) (b 2)) (list a b))
</pre>

<p>You could do any of the following by accident:
</p>

<pre class="indented">
(f 1 :a 2)  ; what is a?
(f :b 1 2)  ; what is b?
(f :c 3)    ; did you really want a to be :c and b to be 3?
</pre>

<p>In the last case, to pass a keyword deliberately, either include the
argument keyword: <code>(f :a :c)</code>, or make the default value a keyword:
<code>(define* (f (a :c) ...))</code>.
To turn off this error check, add :allow-other-keys at the end of the parameter list.
</p>

</div>


<details>
<summary class="indented">rest arg nits</summary>
<div class="indented">

<p>s7's lambda* arglist handling is not the same as CL's lambda-list.  First,
you can have more than one :rest parameter:
</p>

<pre class="indented">
&gt; ((lambda* (:rest a :rest b) (map + a b)) 1 2 3 4 5) 
<em class="gray">'(3 5 7 9)</em>
</pre>

<p>and second, the rest parameter, if any, takes up an argument slot just like any other
argument:
</p>

<pre class="indented">
&gt; ((lambda* ((b 3) :rest x (c 1)) (list b c x)) 32)
<em class="gray">(32 1 ())</em>
&gt; ((lambda* ((b 3) :rest x (c 1)) (list b c x)) 1 2 3 4 5)
<em class="gray">(1 3 (2 3 4 5))</em>
</pre>

<p>CL would agree with the first case if we used &amp;key for 'c', but would give an error in the second.
Of course, the major difference is that s7 keyword arguments don't insist that the key be present.
The :rest argument is needed in cases like these because we can't use an expression
such as:
</p>

<pre class="indented">
&gt; ((lambda* ((a 3) . b c) (list a b c)) 1 2 3 4 5)
<em class="red">error: </em><em class="gray">stray dot?</em>
&gt; ((lambda* (a . (b 1)) b) 1 2) ; the reader turns the arglist into (a b 1)
<em class="red">error: </em><em class="gray">lambda* parameter '1 is a constant</em>
</pre>

<p>Yet another nit: the :rest argument is not considered a keyword argument, so
</p>

<pre class="indented">
&gt; (define* (f :rest a) a)
<em class="gray">f</em>
&gt; (f :a 1)
<em class="gray">(:a 1)</em>
</pre>
</div>
</details>

</blockquote>


<div class="header" id="macros"><h4>macros</h4></div>


<p><em class=def id="definemacro">define-macro</em>, 
<em class=def id="definemacrostar">define-macro*</em>, 
<em class=def id="macroexpand">macroexpand</em>,
<em class=def id="gensym">gensym</em>, 
<em class=def id="gensym?">gensym?</em>, and 
<em class=def id="macrop">macro?</em>
implement the standard old-time macros.  
</p>

<pre class="indented">
&gt; (define-macro (and-let* vars . body)
    `(let () (and ,@(map (lambda (v) `(define ,@v)) vars) (begin ,@body))))

&gt; (define-macro (<em class=def id="trace">trace</em> f)
    `(define ,f 
       (apply lambda 'args 
         `((format () "(~A ~{~A~^ ~}) -&gt; " ',',f args)
           (let ((val (apply ,,f args))) 
             (format () "~A~%" val) 
             val)))))
<em class="gray">trace</em>
&gt; (trace abs)
<em class="gray">abs</em>
&gt; (abs -1.5)
<em class="gray">(abs -1.5) -&gt; 1.5</em>
<em class="gray">1.5</em>
</pre>

<p>macroexpand can help debug a macro.  I always forget that it
wants an expression:
</p>

<pre class="indented">
&gt; (define-macro (add-1 arg) `(+ 1 ,arg))
<em class="gray">add-1</em>
&gt; (macroexpand (add-1 32))
<em class="gray">(+ 1 32)</em>
</pre>

<p>gensym returns a symbol that is guaranteed to be unused.  It takes an optional string argument
giving the new symbol name's prefix.  gensym? returns #t if its argument is a symbol created by gensym.
</p>

<pre class="indented">
(define-macro (pop! sym)
  (let ((v (<em class=red>gensym</em>)))
    `(let ((,v (car ,sym)))
       (set! ,sym (cdr ,sym))
       ,v)))
</pre>

<p>As in define*, the starred forms give optional and keyword arguments:
</p>

<pre class="indented">
&gt; (define-macro* (add-2 a (b 2)) `(+ ,a ,b))
<em class="gray">add-2</em>
&gt; (add-2 1 3)
<em class="gray">4</em>
&gt; (add-2 1)
<em class="gray">3</em>
&gt; (add-2 :b 3 :a 1)
<em class="gray">4</em>
</pre>

<p>A bacro is a macro that expands its body and evaluates
the result in the calling environment. 
</p>

<pre class="indented">
(define setf
  (let ((args (gensym))
        (name (gensym)))
     (apply <em class=red>define-bacro</em> `((,name . ,args)        
			   (unless (null? ,args)
			     (apply set! (car ,args) (cadr ,args) ())
			     (apply setf (cddr ,args)))))))
</pre>


<p>
The setf argument is a gensym (created when setf is defined) so that its name does not shadow any existing
variable.  Bacros expand in the calling environment, and a normal argument name
might shadow something in that environment while the bacro is being expanded.
Similarly, if you introduce bindings in the bacro expansion code, you need to
keep track of which environment you want things to happen in.  Use with-let
and gensym liberally.  
stuff.scm has bacro-shaker which can find inadvertent name collisions,
but it is flighty and easily confused.
See s7test.scm for many examples of macros including such perennial favorites as
loop, dotimes, do*, enum, pushnew, and defstruct.
The calling environment itself is (outlet (curlet)) from within a bacro, so
</p>

<pre class="indented">
(define-bacro (holler)
  `(format *stderr* "(~S~{ ~S ~S~^~})~%" 
	   (let ((f __func__))
	     (if (pair? f) (car f) f))
	   (map (lambda (slot)
		  (values (symbol-&gt;keyword (car slot)) (cdr slot)))		  
		(reverse (map values ,(outlet (curlet)))))))

(define (f1 a b)
  (holler)
  (+ a b))

(f1 2 3) ; prints out "(f1 :a 2 :b 3)" and returns 5
</pre>


<blockquote>

<div class="indented">

<p>
Since a bacro (normally) sheds its define-time environment:
</p>

<pre class="indented">
(define call-bac
  (let ((<em class=red>x</em> 2))
    (define-bacro (m a) `(+ ,a ,<em class=red>x</em>))))

&gt; (call-bac 1) 
<em class="red">error: </em><em class="gray">x: unbound variable</em>
</pre>
<p>
A macro here returns 3.  But don't be hasty!  The bacro can get its define-time environment (its closure)
via funclet, so in fact, define-macro is a special case of define-bacro!  We can define
macros that work in all four ways: the expansion can happen in either the definition or calling environment,
as can the evaluation of that expansion.  In a bacro, both happen in the calling environment
if we take no other action, and in a normal macro (define-macro), the expansion happens in the definition
environment, and the evaluation in the calling environment.
Here's a brief example of all four:
</p>

<pre class="indented">
(let ((x 1) (y 2)) 
  (define-bacro (bac1 a) 
     `(+ ,x y ,a))        ; expand and eval in calling env
  (let ((x 32) (y 64)) 
    (bac1 3)))            ; (with-let (inlet 'x 32 'y 64) (+ 32 y 3))
-&gt; 99                     ;  with-let and inlet refer to <a href="#environments">environments</a>

(let ((x 1) (y 2))        ; this is like define-macro
  (define-bacro (bac2 a) 
    (with-let (sublet (funclet bac2) :a a)
      `(+ ,x y ,a)))      ; expand in definition env, eval in calling env
  (let ((x 32) (y 64)) 
    (bac2 3)))            ; (with-let (inlet 'x 32 'y 64) (+ 1 y 3))
-&gt; 68

(let ((x 1) (y 2))
  (define-bacro (bac3 a) 
    (let ((e (with-let (sublet (funclet bac3) :a a)
	       `(+ ,x y ,a))))
      `(with-let ,(sublet (funclet bac3) :a a)
	 ,e)))           ; expand and eval in definition env 
  (let ((x 32) (y 64)) 
    (bac3 3)))           ; (with-let (inlet 'x 1 'y 2) (+ 1 y 3))
-&gt; 6

(let ((x 1) (y 2))
  (define-bacro (bac4 a) 
    (let ((e `(+ ,x y ,a)))
      `(with-let ,(sublet (funclet bac4) :a a)
	 ,e)))           ; expand in calling env, eval in definition env
  (let ((x 32) (y 64))     
    (bac4 3)))           ; (with-let (inlet 'x 1 'y 2) (+ 32 y 3))
-&gt; 37
</pre>
</div>

<div class="indented">

<p>As the setf example shows, a macro is a first-class citizen of s7.  You can
pass it as a function argument, apply it to a list, return it from a function,
call it recursively, 
and assign it to a variable.  You can even set its procedure-setter!
</p>

<pre class="indented">
&gt; (define-macro (hi a) `(+ ,a 1))
<em class="gray">hi</em>
&gt; (apply hi '(4))
<em class="gray">5</em>
&gt; (define (fmac mac) (apply mac '(4)))
<em class="gray">fmac</em>
&gt; (fmac hi)
<em class="gray">5</em>
&gt; (define (fmac mac) (mac 4))
<em class="gray">fmac</em>
&gt; (fmac hi)
<em class="gray">5</em>
&gt; (define (make-mac)
    (define-macro (hi a) `(+ ,a 1)))
<em class="gray">make-mac</em>
&gt; (let ((x (make-mac)))
    (x 2))
<em class="gray">3</em>
&gt; (define-macro (ref v i) `(vector-ref ,v ,i))
<em class="gray">ref</em>
&gt; (define-macro (set v i x) `(vector-set! ,v ,i ,x))
<em class="gray">set</em>
&gt; (set! (procedure-setter ref) set)
<em class="gray">set</em>
&gt; (let ((v (vector 1 2 3))) (set! (ref v 0) 32) v)
<em class="gray">#(32 2 3)</em>
</pre>

<p>To expand all the macros in a piece of code:
</p>
<pre class="indented">
(define-macro (fully-macroexpand form)
  (list 'quote
    (let expand ((form form))
      (cond ((not (pair? form)) form)
            ((and (symbol? (car form))
                  (macro? (symbol-&gt;value (car form))))
             (expand (apply macroexpand (list form))))
            ((and (eq? (car form) 'set!)  ; look for (set! (mac ...) ...) and use mac's procedure-setter
                  (pair? (cdr form))
                  (pair? (cadr form))
                  (macro? (symbol-&gt;value (caadr form))))
             (expand
              (apply (eval
                 (procedure-source (procedure-setter (symbol-&gt;value (caadr form)))))
                (append (cdadr form) (cddr form)))))
            (else (cons (expand (car form)) (expand (cdr form))))))))
</pre>
<p>This does not always handle bacros correctly because their expansion can depend on the run-time 
state.
</p>
</div>


<details>
<summary class="indented">backquote details</summary>
<div class="indented">

<p>Backquote (quasiquote) in s7 is almost trivial.  Constants are unchanged, symbols are quoted,
",arg" becomes "arg", and ",@arg" becomes "(apply values arg)" &mdash; hooray for real multiple values!
It's almost as easy to write the actual macro body as the backquoted version of it. 
</p>

<pre class="indented">
&gt; (define-macro (hi a) `(+ 1 ,a))
<em class="gray">hi</em>
&gt; (procedure-source hi)
<em class="gray">(lambda (a) ({list} '+ 1 a))</em>

&gt; (define-macro (hi a) `(+ 1 ,@a))
<em class="gray">hi</em>
&gt; (procedure-source hi)
<em class="gray">(lambda (a) ({list} '+ 1 ({apply_values} a)))</em>
</pre>

<p>{list} is a special version of list to avoid name collisions
and handle a few tricky details (similarly for {apply_values}).  There is no unquote-splicing
macro in s7;  ",@(...)" becomes "(unquote ({apply_values} ...))" at read-time.  There shouldn't be any unquote
either.  In Scheme the reader turns ,x into (unquote x), so:
</p>

<pre>
&gt; (let (,'a) unquote)
<em class="gray">a</em>
&gt; (let (, (lambda (x) (+ x 1))) ,,,,'3)
<em class="gray">7</em>
</pre>
<p>comma becomes a sort of symbol macro! I think I'll remove unquote; ,x and ,@x will still work
as expected, but there will not be any "unquote" or "unquote-splicing" in the resultant source code.  Just to make life difficult:
</p>
<pre>
&gt; (let (' 1) quote)
<em class="gray">1</em>
</pre>
<p>but that translation is so ingrained in lisp
that I'm reluctant to change it.  The two unquote names, on the
other hand, seem unnecessary.
</p>
</div>
</details>


<p>s7 macros are not hygienic.  For example,
</p>

<pre class="indented">
&gt; (define-macro (mac b) 
    `(let ((a 12)) 
       (+ a ,b)))
<em class="gray">mac</em>
&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">144</em>
</pre>

<p>This returns 144 because '+' has turned into '*', and 'a' is the internal 'a',
not the argument 'a'.  We get <code>(* 12 12)</code> where we might have expected
<code>(+ 12 1)</code>.  
Starting with the '+' problem, 
as long as the redefinition of '+' is local (that is, it happens after the macro definition), we can unquote the +:
</p>

<pre class="indented">
&gt; (define-macro (mac b) 
    `(let ((a 12)) 
       (,+ a ,b))) ; ,+ picks up the definition-time +
<em class="gray">mac</em>
&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">24                 ; (+ a a) where a is 12</em>
</pre>

<p>But the unquote trick won't work if we have previously loaded some file that redefined '+'
at the top-level (so at macro definition time, + is *, but we want the built-in +).
Although this example is silly, the problem is real in Scheme
because Scheme has no reserved words and only one name space.
</p>

<pre class="indented">
&gt; (define + *)
<em class="gray">+</em>
&gt; (define (add a b) (+ a b))
<em class="gray">add</em>
&gt; (add 2 3)
<em class="gray">6</em>
&gt; (define (divide a b) (/ a b))
<em class="gray">divide</em>
&gt; (divide 2 3)
<em class="gray">2/3</em>
&gt; (set! / -) ; a bad idea &mdash; this turns off s7's optimizer
<em class="gray">-</em>
&gt; (divide 2 3)
<em class="gray">-1</em>
</pre>

<p>Obviously macros are not the problem here.  Since 
we might be loading
code written by others, it's sometimes hard to tell what names
that code depends on or redefines.
We need a way to get the pristine (start-up, built-in) value of '+'.
One long-winded way in s7 uses <a href="#unlet">unlet</a>:
</p>

<pre class="indented">
&gt; (define + *)
<em class="gray">+</em>
&gt; (define (add a b) (with-let (unlet) (+ a b)))
<em class="gray">add</em>
&gt; (add 2 3)
<em class="gray">5</em>
</pre>

<p>But this is hard to read, and it's not inconceivable that we might want all three
values of a symbol, the start-up value, the definition-time value, and the
current value.  The latter can be accessed with the bare symbol, the definition-time
value with unquote (','), and the start-up value with either unlet
or #_&lt;name&gt;.  That is, #_+ is a reader macro for <code>(with-let (unlet) +)</code>.
</p>

<pre class="indented">
&gt; (define-macro (mac b) 
    `(<em class=red>#_let</em> ((a 12)) 
       (<em class=red>#_+</em> a ,b))) ; #_+ and #_let are start-up values
<em class="gray">mac</em>
&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">24                 ; (+ a a) where a is 12 and + is the start-up +</em>

;;; make + generic (there's a similar C-based example below)
&gt; (define (+ . args) 
    (if (null? args) 0 
        (apply (if (number? (car args)) <em class=red>#_+ #_string-append</em>) args)))
<em class="gray">+</em>
&gt; (+ 1 2)
<em class="gray">3</em>
&gt; (+ "hi" "ho")
<em class="gray">"hiho"</em>
</pre>

<p>#_&lt;name&gt; could be implemented via *#readers*:
</p>

<pre class="indented">
(set! *#readers*
  (cons (cons #\_ (lambda (str)
		    (with-let (unlet)
                      (string-&gt;symbol (substring str 1)))))
	*#readers*))
</pre>

<p>
So, now we have only the variable capture problem ('a' has been captured in the preceding examples).
This is the only thing that the gigantic "hygienic macro" systems actually deal with:
a microscopic problem that you'd think, from the hype, was up there with malaria and the
national debt.  gensym is the standard approach:
</p>

<pre class="indented">
&gt; (define-macro (mac b) 
    (let ((var (<em class=red>gensym</em>))) 
      `(#_let ((,var 12))
         (#_+ ,var ,b))))
<em class="gray">mac</em>
&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">13</em>

;; or use lambda:
&gt; (define-macro (mac b) 
  `((lambda (b) (let ((a 12)) (#_+ a b))) ,b))
<em class="gray">mac</em>
&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">13</em>
</pre>

<p>But in s7, the simplest approach uses environments.  You have complete
control over the environment at any point:
</p>

<pre>
(define-macro (mac b)
  `(with-let (inlet 'b ,b)
     (let ((a 12))
       (+ a b))))

&gt; (let ((a 1) (+ *)) (mac a))
<em class="gray">13</em>

(define-macro (mac1 . b)         ; originally `(let ((a 12)) (+ a ,@b ,@b))
  `(with-let (inlet 'e (curlet)) ; this 'e will not collide with the calling env
     (let ((a 12))               ;   nor will 'a (so no gensyms are needed etc)
       (+ a (with-let e ,@b) (with-let e ,@b)))))

&gt; (let ((a 1) (e 2)) (mac1 (display a) (+ a e)))
<em class="gray">18</em>  ; (and it displays "11")

(define-macro (mac2 x)           ; this will use mac2's definition environment for its body
  `(with-let (sublet (funclet mac2) :x ,x)
     (let ((a 12))
       (+ a b x))))              ; a is always 12, b is whatever b happens to be in mac2's env

&gt; (define b 10)                  ; this is mac2's b
<em class="gray">10</em>
&gt; (let ((+ *) (a 1) (b 15)) (mac2 (+ a b)))
<em class="gray">37</em>                               ; mac2 uses its own a (12), b (10), and + (+)
                                 ;   but (+ a b) is 15 because at that point + is *: (* 1 15)
</pre>

<p>Hygienic macros are trivial!  So s7 does not have syntax-rules because it is not needed.
s7's lint.scm will warn you about any such problematic macro expansion, so I'd
say just write macros as simply as possible, then let lint tell you
that it's time to do the with-let shuffle. When that happens, wrap the macro body in
a with-let that captures the current environment, and at each use of a macro argument
wrap it in a with-let that re-establishes that environment. 
</p>

<div class="indented">
<pre>
(define-macro (swap a b) ; assume a and b are symbols
  `(with-let (inlet 'e (curlet) 'tmp ,a)
     (set! (e ',a) (e ',b))
     (set! (e ',b) tmp)))

&gt; (let ((b 1) (tmp 2)) (swap b tmp) (list b tmp))
<em class="gray">(2 1)</em>

(define-macro (swap a b) ; here a and b can be any settable expressions
  `(set! ,b (with-let (inlet 'e (curlet) 'tmp ,a) 
	      (with-let e (set! ,a ,b))
	      tmp)))

&gt; (let ((v (vector 1 2))) (swap (v 0) (v 1)) v)
<em class="gray">#(2 1)</em>
&gt; (let ((tmp (cons 1 2))) (swap (car tmp) (cdr tmp)) tmp)
<em class="gray">(2 . 1)</em>

(set! (procedure-setter swap) (define-macro (set-swap a b c) `(set! ,b ,c)))

&gt; (let ((a 1) (b 2) (c 3) (d 4)) (swap a (swap b (swap c d))) (list a b c d))
<em class="gray">(2 3 4 1)</em>

;;; but this is simpler:
(define-macro (rotate! . args)
  `(set! ,(args (- (length args) 1))
         (with-let (inlet 'e (curlet) 'tmp ,(car args))
	   (with-let e 
	     ,@(map (lambda (a b) `(set! ,a ,b)) args (cdr args)))
	   tmp)))

&gt; (let ((a 1) (b 2) (c 3)) (rotate! a b c) (list a b c))
<em class="gray">(2 3 1)</em>
</pre>
</div>

<p>
If you want the macro's expanded result
to be evaluated in its definition environment:
</p>
<pre>
(let ((a 3))
  (define-macro (mac b)
    `(with-let (inlet 'b ,b (funclet mac))
       (+ a b)))       ; definition-time "a", call-time "b"
  (define-macro (mac-1 b)
    `(+ a ,b))         ; call-time "a" and "b"
  (let ((a 32))
    (list (mac 1) 
	  (mac-1 1))))
</pre>


<!--
(define (tree-quote tree args)
  (if (pair? tree)
      (if (eq? (car tree) 'quote)
	  tree
	  (cons (tree-quote (car tree) args)
		(tree-quote (cdr tree) args)))
      (if (memq tree args)
	  (list 'quote tree)
	  tree)))

(define-macro (define-hacro name-and-args . body)
  (let ((name (car name-and-args))
	(args (cdr name-and-args)))
    `(define-macro ,name-and-args
       (list 'with-let 
	     (list 'inlet ,@(map (lambda (arg)
				   (values (symbol->keyword arg) arg))
				 args))
	     ,@(tree-quote body args)))))

; (define-hacro (mac a b) `(+ ,a ,b))
; (macroexpand (mac 2 3))
; (with-let (inlet :a 2 :b 3) (+ a b))
; (procedure-source mac)
; (lambda (a b) (list 'with-let (list 'inlet :a a :b b) ({list} '+ 'a 'b)))
-->


<div class="indented">

<p>Here is Peter Seibel's wonderful once-only macro:
</p>

<pre class="indented">
(define-macro (once-only names . body)
  (let ((gensyms (map (lambda (n) (gensym)) names)))
    `(let (,@(map (lambda (g) (list g '(gensym))) gensyms))
       `(let (,,@(map (lambda (g n) (list list g n)) gensyms names))
          ,(let (,@(map list names gensyms))
             ,@body)))))
</pre>

<!-- this was:
(define-macro (once-only names . body)
  (let ((gensyms (map (lambda (n) (gensym)) names)))
    `(let (,@(map (lambda (g) `(,g (gensym))) gensyms))
       `(let (,,@(map (lambda (g n) ``(,,g ,,n)) gensyms names))
          ,(let (,@(map (lambda (n g) `(,n ,g)) names gensyms))
             ,@body)))))
-->

<p>From the land of sparkling bacros:
</p>

<pre class="indented">
(define once-only
  (let ((names (gensym))
	(body (gensym)))
    (apply define-bacro `((,(gensym) ,names . ,body)
      `(let (,@(map (lambda (name) `(,name ,(eval name))) ,names))
	 ,@,body)))))
</pre>
<p>Sadly, with-let is simpler.
</p>
</div>

</blockquote>

<!--
 when is (define-macro (f a) `(+ ,a 1)) not the same as (define (f a) (+ a 1))?
   (f (values 2 3))
   (f most-positive-fixnum) but only because the optimizer messes this up
-->


<div class="header" id="pws"><h4>procedure-setter</h4></div>

<pre class="indented">
(<em class=def>procedure-setter</em> proc)
(<em class=def id="dilambda">dilambda</em> proc setter)
</pre>

<p>Each function (or macro!) can have an associated setter, much like defsetf in CL.  As a convenience, there's also
a way to associate the two functions under one name: dilambda.
The setter is called when the procedure is the target of set!  Its last argument is the
value passed to set!:
</p>

<pre class="indented">
&gt; (procedure-setter cadr)
<em class="gray">#f</em>
&gt; (set! (procedure-setter cadr) 
        (lambda (lst val) 
          (set! (car (cdr lst)) val)))
<em class="gray">#&lt;lambda (lst val)&gt;</em>
&gt; (procedure-source (procedure-setter cadr))
<em class="gray">(lambda (lst val) (set! (car (cdr lst)) val))</em>
&gt; (let ((lst (list 1 2 3))) 
    (set! (cadr lst) 4) 
    lst)
<em class="gray">(1 4 3)</em>
</pre>

<p>In some cases, the setter needs to be a macro:
</p>
<pre class="indented">
&gt; (set! (procedure-setter logbit?)
          (define-macro (m var index on) ; here we want to set "var", so we need a macro
	    `(if ,on
	         (set! ,var (logior ,var (ash 1 ,index)))
	         (set! ,var (logand ,var (lognot (ash 1 ,index)))))))
<em class="gray">m</em>
&gt; (define (mingle a b)
    (let ((r 0))
      (do ((i 0 (+ i 1)))
          ((= i 31) r)
        (set! (logbit? r (* 2 i)) (logbit? a i))
        (set! (logbit? r (+ (* 2 i) 1)) (logbit? b i)))))
<em class="gray">mingle</em>
&gt; (mingle 6 3) ; the INTERCAL mingle operator?
<em class="gray">30</em>
</pre>

<blockquote>


<div class="indented">

<p>Here is a pretty example of dilambda:
</p>

<pre class="indented">
(define-macro (c?r path)
  ;; "path" is a list and "X" marks the spot in it that we are trying to access
  ;; (a (b ((c X)))) &mdash; anything after the X is ignored, other symbols are just placeholders
  ;; c?r returns a dilambda that gets/sets X

  (define (X-marks-the-spot accessor tree)
    (if (pair? tree)
	(or (X-marks-the-spot (cons 'car accessor) (car tree))
	    (X-marks-the-spot (cons 'cdr accessor) (cdr tree)))
	(and (eq? tree 'X) accessor)))

  (let ((body 'lst))
    (for-each
     (lambda (f)
       (set! body (list f body)))
     (reverse (X-marks-the-spot () path)))

    `(<em class=red>dilambda</em>
      (lambda (lst) 
	,body)
      (lambda (lst val)
	(set! ,body val)))))

&gt; ((c?r (a b (X))) '(1 2 (3 4) 5))
<em class="gray">3</em>
&gt; (let ((lst (list 1 2 (list 3 4) 5))) 
   (set! ((c?r (a b (X))) lst) 32)
   lst)
<em class="gray">(1 2 (32 4) 5)</em>
&gt; (procedure-source (c?r (a b (X))))
<em class="gray">(lambda (lst) (car (car (cdr (cdr lst)))))</em>
&gt; ((c?r (a b . X)) '(1 2 (3 4) 5))
<em class="gray">((3 4) 5)</em>
&gt; (let ((lst (list 1 2 (list 3 4) 5))) 
   (set! ((c?r (a b . X)) lst) '(32))
   lst)
<em class="gray">(1 2 32)</em>
&gt; (procedure-source (c?r (a b . X)))
<em class="gray">(lambda (lst) (cdr (cdr lst)))</em>
&gt; ((c?r (((((a (b (c (d (e X)))))))))) '(((((1 (2 (3 (4 (5 6)))))))))) 
<em class="gray">6</em>
&gt; (let ((lst '(((((1 (2 (3 (4 (5 6))))))))))) 
    (set! ((c?r (((((a (b (c (d (e X)))))))))) lst) 32) 
    lst)
<em class="gray">(((((1 (2 (3 (4 (5 32)))))))))</em>
&gt; (procedure-source (c?r (((((a (b (c (d (e X)))))))))))
<em class="gray">(lambda (lst) (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (car (car (car lst)))))))))))))))</em>
</pre>	
</div>


<div class="indented">
<p>Speaking of INTERCAL, COME-FROM:
</p>

<pre class="indented">
(define-macro (define-with-goto-and-come-from name-and-args . body)
  (let ((labels ())
	(gotos ())
	(come-froms ()))

    (let collect-jumps ((tree body))
      (when (pair? tree)
	(when (pair? (car tree))
	  (case (caar tree)
	    ((label)     (set! labels (cons tree labels)))
	    ((goto)      (set! gotos (cons tree gotos)))
	    ((come-from) (set! come-froms (cons tree come-froms)))
	    (else (collect-jumps (car tree)))))
	(collect-jumps (cdr tree))))

    (for-each
     (lambda (goto)
       (let* ((name (cadr (cadar goto)))
	      (label (member name labels (lambda (a b) (eq? a (cadr (cadar b)))))))
	 (if label
	     (set-cdr! goto (car label))
	     (error 'bad-goto "can't find label: ~S" name))))
     gotos)
    
    (for-each
     (lambda (from)
       (let* ((name (cadr (cadar from)))
	      (label (member name labels (lambda (a b) (eq? a (cadr (cadar b)))))))
	 (if label
	     (set-cdr! (car label) from)
	     (error 'bad-come-from "can't find label: ~S" name))))
     come-froms)

    `(define ,name-and-args
       (let ((label (lambda (name) #f))
	     (goto (lambda (name) #f))
	     (come-from (lambda (name) #f)))
	 ,@body))))
</pre>
</div>
</blockquote>


<div class="header" id="generalizedset"><h4>applicable objects, generalized set!, generic functions</h4></div>


<p>A procedure with a setter can be viewed as one generalization of set!.  Another
treats objects as having predefined get and set functions.  In s7
lists, strings, vectors, hash-tables, environments, and any cooperating C or Scheme-defined objects
are both applicable and settable.  newLisp calls this implicit indexing, Kawa has it, Gauche implements it
via object-apply, Guile via procedure-with-setter; CL's funcallable instance might be the same idea.
</p>

<p>
In <code>(vector-ref #(1 2) 0)</code>, for example, vector-ref is just a type
declaration.  But in Scheme, type declarations are unnecessary, so we get exactly
the same result from <code>(#(1 2) 0)</code>.  Similarly, <code>(lst 1)</code> is the
same as <code>(list-ref lst 1)</code>, and <code>(set! (lst 1) 2)</code> is the same
as <code>(list-set! lst 1 2)</code>.
I like this syntax:  the less noise, the better!
</p>

<blockquote>

<div class="indented">

<p>Well, maybe applicable strings look weird: <code>("hi" 1)</code> is #\i, but worse,
so is <code>(cond (1 =&gt; "hi"))</code>!  Even though a string, list, or vector is "applicable", it is
not currently considered to be a procedure, so <code>(procedure? "hi")</code> is #f.  map and for-each, however,
accept anything that apply can handle, so
<code>(map #(0 1) '(1 0))</code> is '(1 0).  (On the first call to map in this case, you get the result of
<code>(#(0 1) 1)</code> and so on).
string-&gt;list, vector-&gt;list, and let-&gt;list are <code>(map values object)</code>.
Their inverses are (and always have been) equally trivial.
</p>


<p>The applicable object syntax makes it easy to write generic functions.
For example, s7test.scm has implementations of Common Lisp's sequence functions.
length, copy, reverse, fill!, iterate, map and for-each are generic in this sense (map always returns a list).
</p>

<pre class="indented">
&gt; (map (lambda (a b) (- a b)) (list 1 2) (vector 3 4))
<em class="gray">(5 -3 9)</em>
&gt; (length "hi")
<em class="gray">2</em>
</pre>

<p>
Here's a generic FFT:
</p>

<pre class="indented">
(define* (cfft! data n (dir 1)) ; (complex data)
  (if (not n) (set! n (length data)))
  (do ((i 0 (+ i 1))
       (j 0))
      ((= i n))
    (if (&gt; j i)
	(let ((temp (data j)))
	  (set! (data j) (data i))
	  (set! (data i) temp)))
    (do ((m (/ n 2)))
        ((or (&lt; m 2) 
             (&lt; j m))
         (set! j (+ j m)))
     (set! j (- j m))
     (set! m (/ m 2))))
  (do ((ipow (floor (log n 2)))
       (prev 1)
       (lg 0 (+ lg 1))
       (mmax 2 (* mmax 2))
       (pow (/ n 2) (/ pow 2))
       (theta (complex 0.0 (* pi dir)) (* theta 0.5)))
      ((= lg ipow))
    (let ((wpc (exp theta))
          (wc 1.0))
      (do ((ii 0 (+ ii 1)))
	  ((= ii prev))
	(do ((jj 0 (+ jj 1))
	     (i ii (+ i mmax))
	     (j (+ ii prev) (+ j mmax)))
	    ((&gt;= jj pow))
	  (let ((tc (* wc (data j))))
	    (set! (data j) (- (data i) tc))
	    (set! (data i) (+ (data i) tc))))
	(set! wc (* wc wpc)))
      (set! prev mmax)))
  data)

&gt; (cfft! (list 0.0 1+i 0.0 0.0))
<em class="gray">(1+1i -1+1i -1-1i 1-1i)</em>
&gt; (cfft! (vector 0.0 1+i 0.0 0.0))
<em class="gray">#(1+1i -1+1i -1-1i 1-1i)</em>
</pre>

<p>And a generic function that copies one sequence's elements into another sequence:
</p>
<pre class="indented">
(define (copy-into source dest) ; this is equivalent to (copy source dest)
  (do ((i 0 (+ i 1))) 
      ((= i (min (length source) (length dest))) 
       dest)
    (set! (dest i) (source i))))
</pre>

<p>but that is already built-in as the two-argument version of the copy function.
</p>
</div>


<div class="indented">

<p>There is one place where list-set! and friends are not the same as set!: the former
evaluate their first argument, but set! does not (with a quibble; see below):
</p>

<pre class="indented">
&gt; (let ((str "hi")) (string-set! (let () str) 1 #\a) str)
<em class="gray">"ha"</em>
&gt; (let ((str "hi")) (set! (let () str) 1 #\a) str)
<em class="gray">;((let () str) 1 #\a): too many arguments to set!</em>
&gt; (let ((str "hi")) (set! ((let () str) 1) #\a) str)
<em class="gray">"ha"</em>
&gt; (let ((str "hi")) (set! (str 1) #\a) str)
<em class="gray">"ha"</em>
</pre>

<p>set! looks at its first argument to decide what to set.
If it's a symbol, no problem.  If it's a list, set! looks at its car to see if it is
some object that has a setter.  If the car is itself a list, set! evaluates the internal
expression, and tries again.  So the second case above is the only one that won't work.
And of course:
</p>

<pre class="indented">
&gt; (let ((x (list 1 2))) 
    (set! ((((lambda () (list x))) 0) 0) 3) 
    x) 
<em class="gray">(3 2)</em>
</pre>
</div>


<div class="indented">

<p>By my count, around 20 of the Scheme built-in functions are already generic in the sense
that they accept arguments of many types (leaving aside the numeric and type checking functions, take for example equal?, display,
member, assoc, apply, eval, quasiquote, and values).  s7 extends that list with map, for-each, reverse,
and length, and adds a few others such as copy, fill!, sort!, object-&gt;string, object-&gt;let, and append.
newLisp takes a more radical approach than s7: it extends operators such as '&gt;' 
to compare strings and lists, as well as numbers.  In map and for-each, however, you can mix the argument
types, so I'm not as attracted to making '&gt;' generic; you can't, for example, <code>(&gt; "hi" 32.1)</code>,
or even <code>(&gt; 1 0+i)</code>.
</p>
</div>
</blockquote>

<div class="separator"></div>

<p>The somewhat non-standard generic sequence functions in s7 are:
</p>

<pre class="indented">
(<em class=def id="sortb">sort!</em> sequence less?)
(<em class=def id="reverseb">reverse!</em> sequence) and (reverse sequence)
(<em class=def id="fillb">fill!</em> sequence value (start 0) end)
(<em class=def id="s7copy">copy</em> obj) and (copy source destination (start 0) end)
(<em class=def id="objecttostring">object-&gt;string</em> obj)
(object-&gt;let obj)
(length obj)
(append . sequences)
(map func . sequences) and (for-each func . sequences)
(<a href="#morallyequalp">morally-equal?</a> obj1 obj2)
</pre>

<p><b>reverse!</b> is an in-place version of reverse.  That is, 
it modifies the sequence passed to it in the process of reversing its contents.  
If the sequence is a list, remember to use set!:
<code>(set! p (reverse! p))</code>.  This is somewhat inconsistent with other cases,
but historically, lisp programmers have treated the in-place reverse as the fast
version, so s7 follows suit.
</p>

<p>Leaving aside the weird list case,
<b>append</b> returns a sequence of the same type as its first argument.
</p>

<pre class="indented">
&gt; (append #(1 2) '(3 4))
<em class="gray">#(1 2 3 4)</em>
&gt; (append (float-vector) '(1 2) (byte-vector 3 4))
<em class="gray">(float-vector 1.0 2.0 3.0 4.0)</em>
</pre>

<p>
<b>sort!</b> sorts a sequence using the
function passed as its second argument:
</p>

<pre class="indented">
&gt; (sort! (list 3 4 8 2 0 1 5 9 7 6) &lt;)
<em class="gray">(0 1 2 3 4 5 6 7 8 9)</em>
</pre>

<p>Underlying some of these functions are generic iterators, also built-into s7:
</p>

<pre class="indented">
(<em class=def id="makeiterator">make-iterator</em> sequence)
(<em class=def id="iteratorp">iterator?</em> obj)
(<em class=def id="iterate">iterate</em> iterator)
(<em class=def id="iteratorsequence">iterator-sequence</em> iterator)
(<em class=def id="iteratoratend">iterator-at-end?</em> iterator)
</pre>

<p><b>make-iterator</b> takes a sequence argument and returns an iterator object that traverses
that sequence as it is called.  The iterator itself can be treated as a function of no arguments,
or (for code clarity) it can be the argument to <b>iterate</b>, which does the same thing.
That is <code>(iter)</code> is the same as <code>(iterate iter)</code>.  The sequence that an iterator is traversing
is <b>iterator-sequence</b>.
</p>
<p>
If the sequence is a hash-table or let, the iterator normally returns a cons of the key and value.
There are many cases where this overhead is objectionable, so make-iterator takes a third optional
argument, the cons to use (changing its car and cdr directly on each call).
</p>

<p>When an iterator reaches the end of its sequence, it returns #&lt;eof&gt;.  It used to
return nil; I can't decide whether this change is an improvement.  If an iterator over a
list notices that its list is circular, it returns #&lt;eof&gt;.  map and for-each use
iterators, so if you pass a circular list to either, it will stop eventually.  (An
arcane consequence for method writers: specialize make-iterator, not map or for-each).
</p>

<pre class="indented">
(define (find-if f sequence)
  (let ((iter (make-iterator sequence)))
    (do ((x (iter) (iter)))
	((or (eof-object? x) (f x))
	 (and (not (eof-object? x)) x)))))
</pre>

<p>But of course a sequence might contain #&lt;eof&gt;! So to be really safe, use iterator-at-end? 
instead of eof-object?.
</p>

<p>The argument to make-iterator can also be a function or macro.  
There should be a variable named 'iterator with a non-#f
value in the closure's environment:
</p>
<pre class="indented">
(define (make-circular-iterator obj)
  (let ((iter (make-iterator obj)))
    (make-iterator 
     (let ((iterator? #t))
       (lambda ()
         (let ((result (iter)))
	   (if (eof-object? result)
	       ((set! iter (make-iterator obj)))
	       result)))))))
</pre>
<p>The 'iterator? variable is similar to the 'documentation variable used by procedure-documentation.
It gives make-iterator some hope of catching inadvertent bogus function arguments that would
otherwise cause an infinite loop. 
</p>


<div class="header" id="multidimensionalvectors"><h4>multidimensional vectors</h4></div>


<p>
s7 supports
vectors with any number of dimensions.  It is here, in particular, that generalized
set! shines.  make-vector's second argument can be a list of dimensions, rather than
an integer as in the one dimensional case:
</p>

<pre class="indented">
(make-vector (list 2 3 4))
(make-vector '(2 3) 1.0)
(vector-dimensions (make-vector '(2 3 4))) -&gt; (2 3 4)
</pre>

<p>The second example includes the optional initial element.
<code>(vect i ...)</code> or <code>(vector-ref vect i ...)</code> return the given
element, and <code>(set! (vect i ...) value)</code> and <code>(vector-set! vect i ... value)</code>
set it.  vector-length (or just length) returns the total number of elements.
vector-dimensions returns a list of the dimensions.
</p>

<pre class="indented">
&gt; (define v (make-vector '(2 3) 1.0))
<em class="gray">#2D((1.0 1.0 1.0) (1.0 1.0 1.0))</em>
&gt; (set! (v 0 1) 2.0)
<em class="gray">#2D((1.0 2.0 1.0) (1.0 1.0 1.0))</em>
&gt; (v 0 1)
<em class="gray">2.0</em>
&gt; (vector-length v)
<em class="gray">6</em>
</pre>

<p>This function initializes each element of a multidimensional vector:
</p>

<pre class="indented">
(define (make-array dims . inits)
  (make-shared-vector (apply vector (flatten inits)) dims))

&gt; (make-array '(3 3) '(1 1 1) '(2 2 2) '(3 3 3))
<em class="gray">#2D((1 1 1) (2 2 2) (3 3 3))</em>
</pre>

<p>make-int-vector and make-float-vector produce homogeneous vectors holding
s7_ints or s7_doubles.
These are mostly useful in conjunction with C code.  These
homogeneous vector functions are currently built-in:
</p>

<pre class="indented">
(<em class=def>float-vector?</em> obj)
(<em class=def>float-vector</em> . args)
(<em class=def>make-float-vector</em> len (init 0.0))
(<em class=def>float-vector-ref</em> obj . indices)
(<em class=def>float-vector-set!</em> obj indices[...] value)

(<em class=def id="intvectorp">int-vector?</em> obj)
(<em class=def id="intvector">int-vector</em> . args)
(<em class=def id="makeintvector">make-int-vector</em> len (init 0))
(<em class=def id="intvectorref">int-vector-ref</em> obj . indices)
(<em class=def id="intvectorset">int-vector-set!</em> obj indices[...] value)
</pre>

<div class="indented">
<p>And also, for completeness:</p>
<pre class="indented">
(<em class=def id="bytevectorp">byte-vector?</em> obj)
(<em class=def id="bytevector">byte-vector</em> . args)
(<em class=def id="makebytevector">make-byte-vector</em> len (init 0))
(<em class=def id="stringtobytevector">string-&gt;byte-vector</em> str)
</pre>
<p>but these are really just strings in disguise.</p>
</div>

<p>To access a vector's elements with different dimensions than the original had, use 
<code>(make-shared-vector original-vector new-dimensions (offset 0))</code>:
</p>

<pre class="indented">
&gt; (let ((v1 #2d((1 2 3) (4 5 6)))) 
    (let ((v2 (make-shared-vector v1 '(6)))) ; flatten the original
      v2))
<em class="gray">#(1 2 3 4 5 6)</em>
&gt; (let ((v1 #(1 2 3 4 5 6))) 
    (let ((v2 (make-shared-vector v1 '(3 2)))) 
      v2))
<em class="gray">#2D((1 2) (3 4) (5 6))</em>
</pre>
<blockquote>

<div class="indented">

<p>matrix multiplication:
</p>

<pre>
(define (matrix-multiply A B)
  ;; assume square matrices and so on for simplicity
  (let ((size (car (vector-dimensions A))))
    (do ((C (make-vector (list size size) 0))
         (i 0 (+ i 1)))
	((= i size) C)
      (do ((j 0 (+ j 1)))
	  ((= j size))
	(do ((sum 0)
             (k 0 (+ k 1)))
	    ((= k size)
             (set! (C i j) sum))
	  (set! sum (+ sum (* (A i k) (B k j)))))))))
</pre>
</div>


<div class="indented">

<p>Conway's game of Life:
</p>

<pre>
(define* (life (width 40) (height 40))
  (let ((state0 (make-vector (list width height) 0))
	(state1 (make-vector (list width height) 0)))

    ;; initialize with some random pattern
    (do ((x 0 (+ x 1)))
	((= x width))
      (do ((y 0 (+ y 1)))
	  ((= y height))
	(set! (state0 x y) (if (&lt; (random 100) 15) 1 0))))

    (do () ()
      ;; show current state (using terminal escape sequences, borrowed from the Rosetta C code)
      (format *stderr* "~C[H" #\escape)           ; ESC H = tab set
      (do ((y 0 (+ y 1)))
	  ((= y height))
	(do ((x 0 (+ x 1)))
	    ((= x width))
	  (format *stderr*
                  (if (zero? (state0 x y))
	              "  "                        ; ESC 07m below = inverse
	              (values "~C[07m  ~C[m" #\escape #\escape))))
	(format *stderr* "~C[E" #\escape))        ; ESC E = next line

      ;; get the next state
      (do ((x 1 (+ x 1)))
	  ((= x (- width 1)))
	(do ((y 1 (+ y 1)))
	    ((= y (- height 1)))
	  (let ((n (+ (state0 (- x 1) (- y 1))
		      (state0    x    (- y 1))
		      (state0 (+ x 1) (- y 1))
		      (state0 (- x 1)    y)      
		      (state0 (+ x 1)    y)      
		      (state0 (- x 1) (+ y 1))
		      (state0    x    (+ y 1))
		      (state0 (+ x 1) (+ y 1)))))
	    (set! (state1 x y) 
		  (if (or (= n 3) 
			  (and (= n 2)
			       (not (zero? (state0 x y)))))
		      1 0)))))
      (do ((x 0 (+ x 1)))
	  ((= x width))
	(do ((y 0 (+ y 1)))
	    ((= y height))
	  (set! (state0 x y) (state1 x y)))))))
</pre>
</div>


<div class="indented">

<p>Multidimensional vector constant syntax is modelled after CL: #nd(...) or #nD(...) 
signals that the lists specify the elements of an 'n' dimensional vector: <code>#2D((1 2 3) (4 5 6))</code>
</p>

<pre class="indented">
&gt; (vector-ref #2D((1 2 3) (4 5 6)) 1 2)
<em class="gray">6</em>
&gt; (matrix-multiply #2d((-1 0) (0 -1)) #2d((2 0) (-2 2)))
<em class="gray">#2D((-2 0) (2 -2))</em>
</pre>

<p>If any dimension has 0 length, you get an n-dimensional empty vector.  It is not
equal to a 1-dimensional empty vector.
</p>

<pre class="indented">
&gt; (make-vector '(10 0 3))
<em class="gray">#3D()</em>
&gt; (equal? #() #3D())
<em class="gray">#f</em>
</pre>
</div>


<div class="indented">

<p>To save on costly parentheses, and make it easier to write generic multidimensional sequence functions,
you can use this same syntax with lists.
</p>

<pre class="indented">
&gt; (let ((L '((1 2 3) (4 5 6))))
    (L 1 0))              ; same as (list-ref (list-ref L 1) 0) or ((L 1) 0)
<em class="gray">4</em>
&gt; (let ((L '(((1 2 3) (4 5 6)) ((7 8 9) (10 11 12))))) 
    (set! (L 1 0 2) 32)   ; same as (list-set! (list-ref (list-ref L 1) 0) 2 32) which is unreadable!
    L)
<em class="gray">(((1 2 3) (4 5 6)) ((7 8 32) (10 11 12)))</em>
</pre>

<p>Or with vectors of vectors, of course:
</p>

<pre class="indented">
&gt; (let ((V #(#(1 2 3) #(4 5 6)))) 
    (V 1 2))              ; same as (vector-ref (vector-ref V 1) 2) or ((V 1) 2)
<em class="gray">6</em>
&gt; (let ((V #2d((1 2 3) (4 5 6))))
    (V 0))
<em class="gray">#(1 2 3)</em>
</pre>

<p>There's one difference between a vector-of-vectors and a multidimensional vector:
in the latter case, you can't clobber one of the inner vectors. 
</p>

<pre class="indented">
&gt; (let ((V #(#(1 2 3) #(4 5 6)))) (set! (V 1) 32) V)
<em class="gray">#(#(1 2 3) 32)</em>
&gt; (let ((V #2d((1 2 3) (4 5 6)))) (set! (V 1) 32) V)
<em class="gray">;not enough args for vector-set!: (#2D((1 2 3) (4 5 6)) 1 32)</em>
</pre>
</div>


<div class="indented">

<p>Using lists to display the inner vectors may not be optimal, especially when the elements are also lists:
</p>

<pre class="indented">
#2D(((0) (0) ((0))) ((0) 0 ((0))))
</pre>

<p>The "#()" notation is no better (the elements can be vectors), and I'm not a fan of "[]" parentheses.
Perhaps we could use different colors?  Or different size parentheses?
</p>

<pre class="indented">
#2D<em class=green>(</em><em class=red>(</em>(0) (0) ((0))<em class=red>)</em> <em class=red>(</em>(0) 0 ((0))<em class=red>)</em><em class=green>)</em>
#2D<em class="bigger">(</em><em class="big">(</em>(0) (0) ((0))<em class="big">)</em> <em class="big">(</em>(0) 0 ((0))<em class="big">)</em><em class="bigger">)</em>
</pre>

<p>A similar problem afflicts homogeneous vectors.  We need some reasonable way to express
such a vector even when it has more than one dimension.  My first thought was <code>#(...)#</code>,
but that makes <code>(let ((b1 0)) (#(1 2)#b1))</code> ambiguous.
</p>

</div>


<div class="indented">

<p>I'm not sure how to handle vector-&gt;list and list-&gt;vector in the multidimensional case.
Currently, vector-&gt;list flattens the vector, and list-&gt;vector always returns a
one dimensional vector, so the two are not inverses.
</p>

<pre class="indented">
&gt; (vector-&gt;list #2d((1 2) (3 4)))
<em class="gray">(1 2 3 4)</em>             ; should this be '((1 2) (3 4)) or '(#(1 2) #(3 4))?
&gt; (list-&gt;vector '(#(1 2) #(3 4))) ; what about '((1 2) (3 4))?
<em class="gray">#(#(1 2) #(3 4))      </em>
</pre>

<p>
This also affects format and sort!:
</p>

<pre class="indented">
&gt; (format #f "~{~A~^ ~}" #2d((1 2) (3 4)))
<em class="gray">"1 2 3 4"</em>
&gt; (sort! #2d((1 4) (3 2)) &gt;) 
<em class="gray">#2D((4 3) (2 1))</em>
</pre>

<p>Perhaps make-shared-vector can help:
</p>

<pre class="indented">
&gt;(make-shared-vector (list-&gt;vector '(1 2 3 4)) '(2 2))
<em class="gray">#2D((1 2) (3 4))</em>
</pre>

</div>

<div class="indented">

<p>Another question: should we accept the multi-index syntax in a case such as <code>
(#("abc" "def") 0 2)</code>?
My first thought was that the indices should all refer to the same
type of object, so s7 would complain in a mixed case like that.
If we can nest any applicable objects and apply the whole thing to
an arbitrary list of indices, ambiguities arise:
</p>

<pre class="indented">
((lambda (x) x) "hi" 0) 
((lambda (x) (lambda (y) (+ x y))) 1 2)
</pre>

<p>I think these should complain that the function got too many arguments,
but from the implicit indexing point of view, they could be interpreted
as:
</p>

<pre class="indented">
(string-ref ((lambda (x) x) "hi") 0) ; i.e. (((lambda (x) x) "hi") 0)
(((lambda (x) (lambda (y) (+ x y))) 1) 2)
</pre>

<p>Add optional and rest arguments, and you can't tell who is supposed to
take which arguments.  
Currently, you can mix types with implicit indices,
but a function grabs all remaining indices.  Trickier than I expected!
</p>

</div>

</blockquote>


<div class="header" id="hashtables"><h4>hash-tables</h4></div>


<ul>
<li>(<em class=def id="makehashtable">make-hash-table</em> (size 8) eq-func)
<li>(<em class=def id="hashtable">hash-table</em> ...)
<li>(<em class=def id="hashtablestar">hash-table*</em> ...)
<li>(<em class=def id="hashtablep">hash-table?</em> obj)
<li>(<em class=def id="hashtableref">hash-table-ref</em> ht key)
<li>(<em class=def id="hashtableset">hash-table-set!</em> ht key value)
<li>(<em class=def id="hashtableentries">hash-table-entries</em> ht)
</ul>

<p>
Each hash-table keeps track of the keys it contains, optimizing the search wherever possible.
Any s7 object can be the key or the key's value.
If you pass a table size that is not a power of 2, make-hash-table rounds it up to the next power of 2.
The table grows as needed.  length returns the current size.
If a key is not in the table, hash-table-ref returns #f.  To remove a key,
set its value to #f; to remove all keys, <code>(fill! table #f)</code>.
</p>

<pre class="indented">
&gt; (let ((ht (make-hash-table)))
    (set! (ht "hi") 123)
    (ht "hi"))
<em class="gray">123</em>
</pre>

<p>hash-table (the function) parallels the functions vector, list, and string.  Its arguments are conses containing key/value pairs.
The result is a new hash-table with those values preinstalled: <code>(hash-table '("hi" . 32) '("ho" . 1))</code>.
After much use, I now think it is more convenient here, as in inlet, to use hash-table*; its arguments are
simply the keys and values, without the consing: <code>(hash-table* 'a 1 'b 2)</code>.
Implicit indexing gives multilevel hashes:
</p>

<pre class="indented">
&gt; (let ((h (hash-table* 'a (hash-table* 'b 2 'c 3)))) (h 'a 'b))
<em class="gray">2</em>
&gt; (let ((h (hash-table* 'a (hash-table* 'b 2 'c 3)))) (set! (h 'a 'b) 4) (h 'a 'b))
<em class="gray">4</em>
</pre>

<blockquote>

<div class="indented">

<p>Since hash-tables accept the same applicable-object syntax that vectors use, we can 
treat a hash-table as, for example, a sparse array:
</p>

<pre class="indented">
&gt; (define make-sparse-array make-hash-table)
<em class="gray">make-sparse-array</em>
&gt; (let ((arr (make-sparse-array)))
   (set! (arr 1032) "1032")
   (set! (arr -23) "-23")
   (list (arr 1032) (arr -23)))
<em class="gray">("1032" "-23")</em>
</pre>
</div>


<div class="indented">

<p>map and for-each accept hash-table arguments. On each iteration, the map or for-each function is passed
an entry, <code>'(key . value)</code>, in whatever order the entries are encountered in the table. 
</p>

<pre class="indented">
(define (hash-table-&gt;alist table)
  (map values table))

(define (merge-hash-tables . tables) ; probably faster: (define merge-hash-tables append)
  (apply hash-table (apply map values hash-tables)))
</pre>

<p>reverse of a hash-table returns a new table with the keys and values reversed.
fill! sets all the values.
Two hash-tables are equal if they have the same keys with the same values.  This is independent
of the table sizes, or the order in which the key/value pairs were added.
</p>
</div>

<div class="indented">
<p>The third argument to make-hash-table (eq-func) is slightly complicated.  If it is omitted,
s7 chooses the hashing equality and mapping functions based on the keys in the hash-table.
There are times when you know
in advance what equality function you want.  If it's one of the built-in s7 equality
functions, eq?, eqv?, equal?, morally-equal?, =, string=?, string-ci=?, char=?, or char-ci=?,
you can pass that function as the third argument.  In any other case, you need to
give s7 both the equality function and the mapping function. The latter takes any object
and returns the hash-table location for it (an integer).  The problem here is that
for the arbitrary equality function to work, objects that are equal according to that
function have to be mapped to the same hash-table location.  There's no way for s7 to intuit
what this mapping should be except in the built-in cases.  So to specify some arbitrary function, the third
argument is a cons: '(equality-checker mapper).
</p>

<p>Here's a brief example.  In CLM, we have c-objects of type mus-generator (from s7's point of view),
and we want to hash them using equal? (which will call the generator-specific equality function).
But s7 doesn't realize that the mus-generator type covers 40 or 50 internal types, so as the mapper we pass mus-type:
<code>(make-hash-table 64 (cons equal? mus-type))</code>.
</p>
</div>

<div class="indented">
<p>If the hash key is a float (a non-rational number), hash-table-ref uses <a href="#morallyequalp">morally-equal?</a>.
Otherwise, for example, you could use NaN as a key, but then never be able to access it!
</p>
</div>


<div class="indented">
<pre>
(define-macro (define-memoized name&amp;arg . body)
  (let ((arg (cadr name&amp;arg))
	(memo (gensym "memo")))
    `(define ,(car name&amp;arg)
       (let ((,memo (<em class=red>make-hash-table</em>)))
	 (lambda (,arg)
	   (or (,memo ,arg)                             ; check for saved value
	       (set! (,memo ,arg) (begin ,@body)))))))) ; set! returns the new value

&gt; (define (fib n) 
  (if (&lt; n 2) n (+ (fib (- n 1)) (fib (- n 2)))))
<em class="gray">fib</em>
&gt; (define-memoized 
   (memo-fib n) 
     (if (&lt; n 2) n (+ (memo-fib (- n 1)) (memo-fib (- n 2)))))
<em class="gray">memo-fib</em>
&gt; (time (fib 34))         ; un-memoized time
<em class="gray">1.168</em>                        ;   0.70 on ccrma's i7-3930 machines
&gt; (time (memo-fib 34))    ; memoized time
<em class="gray">3.200e-05</em>
&gt; (outlet (funclet memo-fib))
<em class="gray">(inlet '{memo}-18 (hash-table 
    '(0 . 0) '(1 . 1) '(2 . 1) '(3 . 2) '(4 . 3) '(5 . 5) 
    '(6 . 8) '(7 . 13) '(8 . 21) '(9 . 34) '(10 . 55) '(11 . 89) 
    '(12 . 144) '(13 . 233) '(14 . 377) '(15 . 610) '(16 . 987) 
    '(17 . 1597) '(18 . 2584) '(19 . 4181) '(20 . 6765) '(21 . 10946) 
    '(22 . 17711) '(23 . 28657) '(24 . 46368) '(25 . 75025) '(26 . 121393) 
    '(27 . 196418) '(28 . 317811) '(29 . 514229) '(30 . 832040) '(31 . 1346269) 
    '(32 . 2178309) '(33 . 3524578) '(34 . 5702887)))</em>
</pre>
</div>

</blockquote>


<div class="header" id="environments"><h4>environments</h4></div>


<p>An environment holds symbols and their values.  The global environment, for example,
holds all the variables that are defined at the top level.
Environments are first class (and applicable) objects in s7.  
</p>

<pre class="indented">
(<em class=def id="rootlet">rootlet</em>)               the top-level (global) environment
(<em class=def id="curlet">curlet</em>)                the current (innermost) environment
(<em class=def id="funclet">funclet</em> proc)          the environment at the time when proc was defined
(owlet)                 the environment at the point of the last error
(<em class=def id="unlet">unlet</em>)                 the current environment, but all built-in functions have their original values

(<em class=def id="letref">let-ref</em> env sym)       get value of sym in env, same as (env sym)
(<em class=def id="letset">let-set!</em> env sym val)

(<em class=def id="inlet">inlet</em> . bindings)       make a new environment with the given bindings
(<em class=def id="sublet">sublet</em> env . bindings)  same as inlet, but the new environment is local to env
(<em class=def id="varlet">varlet</em> env . bindings)  add new bindings directly to env
(<em class=def id="cutlet">cutlet</em> env . fields)    remove bindings from env

(<em class=def id="letp">let?</em> obj)               #t if obj is an environment
(<em class=def id="with-let">with-let</em> env . body)    evaluate body in the environment env 
(<em class=def id="outlet">outlet</em> env)             the environment that encloses the environment env (settable)
(<em class=def id="lettolist">let-&gt;list</em> env)          return the environment bindings as a list of (symbol . value) cons's

(<em class=def id="openlet">openlet</em> env)            mark env as open (see below)
(<em class=def id="openletp">openlet?</em> env)           #t is env is open
(<em class=def id="coverlet">coverlet</em> env)           mark env as closed (undo an earlier openlet)

(<em class=def id="objecttolet">object-&gt;let</em> obj)        return an environment containing information about obj 
(<em class=def id="lettemporarily">let-temporarily</em> vars . body)
</pre>

<br>
<blockquote>
<pre class="indented">
&gt; (inlet 'a 1 'b 2)
<em class="gray">(inlet 'a 1 'b 2)</em>
&gt; (let ((a 1) (b 2)) (curlet))
<em class="gray">(inlet 'a 1 'b 2)</em>
&gt; (let ((x (inlet :a 1 :b 2))) (x 'a))
<em class="gray">1</em>
&gt; (with-let (inlet 'a 1 'b 2) (+ a b))
<em class="gray">3</em>
&gt; (let ((x (inlet :a 1 :b 2))) (set! (x 'a) 4) x)
<em class="gray">(inlet 'a 4 'b 2)</em>
&gt; (let ((x (inlet))) (varlet x 'a 1) x)
<em class="gray">(inlet 'a 1)</em>
&gt; (let ((a 1)) (let ((b 2)) (outlet (curlet))))
<em class="gray">(inlet 'a 1)</em>
&gt; (let ((e (inlet 'a (inlet 'b 1 'c 2)))) (e 'a 'b)) ; in C terms, e-&gt;a-&gt;b 
<em class="gray">1</em>  
&gt; (let ((e (inlet 'a (inlet 'b 1 'c 2)))) (set! (e 'a 'b) 3) (e 'a 'b))
<em class="gray">3</em>  
</pre>
</blockquote>


<p>As the names suggest, in s7 an environment is viewed as a disembodied let.  Environments are equal if they
contain the same symbols with the same values leaving aside shadowing, and taking into account the environment
chain up to the rootlet.  That is, two environments are equal if any local variable of either has the same value in both.
</p>

<p><b>with-let</b> evaluates its body in the given environment, so
<code>(with-let e . body)</code> is equivalent to
<code>(eval `(begin ,@body) e)</code>, but probably faster.
Similarly, <code>(let bindings . body)</code> is equivalent to
<code>(eval `(begin ,@body) (apply inlet (flatten bindings)))</code>,
ignoring the outer (enclosing) environment (the default outer environment
of inlet is rootlet).
Or better,
</p>
<pre class="indented">
(define-macro (with-environs e . body) 
  `(apply let (map (lambda (a) (list (car a) (cdr a))) ,e) '(,@body)))
</pre>
<p>Or turning it around,</p>
<pre>
(define-macro (Let vars . body)
  `(with-let (sublet (curlet) 
	       ,@(map (lambda (var)
			(values (symbol-&gt;keyword (car var)) (cadr var)))
		      vars))
     ,@body))

(Let ((c 4))
  (Let ((a 2)
        (b (+ c 2)))
  (+ a b c)))
</pre>


<p><b>let-temporarily</b> (now built-into s7) is somewhat similar to fluid-let in other Schemes.
Its syntax looks like
let, but it first saves the current value, then sets the
variable to the new value (via set!), calls the body, and finally restores the
original value.  It can handle anything settable:
</p>
<pre class="indented">
(let-temporarily (((*s7* 'print-length) 8)) (display x))
</pre>
<p>This sets s7's print-length variable to 8 while displaying x, then
puts it back to its original value.
</p>


<p>
<b>sublet</b> adds bindings (symbols with associated values) to an environment.
It does not change the environment passed to it, but
just prepends the new bindings, shadowing any old ones,
as if you had called "let".
To add the bindings directly to the environment,
use <b>varlet</b>.  Both of these functions accept nil as the
'env' argument as shorthand for <code>(rootlet)</code>.
Both also accept other environments as well as individual bindings,
adding all the argument's bindings to the new environment.
<b>inlet</b> is very similar, but normally omits the environment argument.
The arguments to sublet and inlet can be passed as
symbol/value pairs, as a cons, or using keywords as if in define*.
inlet can also be used to copy an environment without accidentally invoking
that environment's copy method.
</p>

<p>Here's an example: we want to define two functions that share a
local variable:
</p>

<pre class="indented">
(varlet (curlet)            ; import f1 and f2 into the current environment
  (let ((x 1))              ; x is our local variable
    (define (f1 a) (+ a x)) 
    (define (f2 b) (* b x)) 
    (inlet 'f1 f1 'f2 f2))) ; export f1 and f2
</pre>

<p>One way to add reader and writer functions to an individual environment slot is:
</p>

<pre class="indented">
(define e (inlet 
            'x (let ((local-x 3)) ; x's initial value
		 (dilambda
		   (lambda () local-x)
		   (lambda (val) (set! local-x (max 0 (min val 100))))))))
&gt; ((e 'x))
<em class="gray">3</em>
&gt; (set! ((e 'x)) 123)
<em class="gray">100</em>
</pre>

<blockquote>
<div class="indented">
<p>I originally used a bunch of foolishly pompous names for the environment functions.
Two are still available for backwards compatibility:
</p>

<pre class="indented">
rootlet      global-environment
curlet       current-environment
</pre>
</div>
</blockquote>


<p>It is possible in Scheme to redefine built-in functions such as car.
To ensure that some code sees the original built-in function definitions,
wrap it in <code>(with-let (unlet) ...)</code>:
</p>
<pre class="indented">
&gt; (let ((caar 123)) 
    (+ caar (with-let (unlet) 
              (caar '((2) 3)))))
<em class="gray">125</em>
</pre>

<p>
with-let and unlet are constants, so you can
use them in any context without worrying about whether they've been redefined.
As mentioned in the macro section, #_&lt;name&gt; is a built-in reader macro
for <code>(with-let (unlet) &lt;name&gt;)</code>,
so for example, #_+ is the built-in + function, no matter what.
<code>(unlet)</code> cleans up the current environment whenever it's called,
so you can use it to revert the REPL. (The environment of built-in functions
that unlet accesses is not accessible from scheme code, so there's no way 
that those values can be clobbered).
</p>

<blockquote>

<div class="indented">

<p>
I think these functions can implement the notions of libraries,
separate namespaces, or modules.  
Here's one way: first the library writer just writes his library.
The normal user simply loads it.  The abnormal user worries about everything,
so first he loads the library in a local let to make sure no bindings escape 
to pollute his code, and then he
uses unlet to
make sure that none of his bindings pollute the library code:
</p>

<pre class="indented">
(let ()
  (with-let (unlet)
    (load "any-library.scm" (curlet)) 
    ;; by default load puts stuff in the global environment
    ...))
</pre>

<p>Now Abnormal User can do what he wants with the library entities.
Say he wants to use "lognor" under the name "bitwise-not-or", and
all the other functions are of no interest:
</p>

<pre class="indented">
(varlet (curlet)
  'bitwise-not-or (with-let (unlet)
                    (load "any-library.scm" (curlet))
                    lognor)) ; lognor is presumably defined in "any-library.scm"
</pre>

<p>Say he wants to make sure the library is cleanly loaded, but all
its top-level bindings are imported into the current environment:
</p>

<pre class="indented">
(varlet (curlet)
  (with-let (unlet)
    (let ()
      (load "any-library.scm" (curlet))
      (curlet)))) ; these are the bindings introduced by loading the library
</pre>

<p>To do the same thing, but prepend "library:" to each name:
</p>

<pre class="indented">
(apply varlet (curlet)
  (with-let (unlet)
    (let ()
      (load "any-library.scm" (curlet))
      (map (lambda (binding)
	     (cons (symbol "library:" (symbol-&gt;string (car binding)))
		   (cdr binding)))
	    (curlet)))))
</pre>

<p>That's all there is to it!  Here is the same idea as a macro:
</p>

<pre>
(define-macro (let! init end . body)
  ;; syntax mimics 'do: (let! (vars&amp;values) ((exported-names) result) body)
  ;;   (let! ((a 1)) ((hiho)) (define (hiho x) (+ a x)))
  `(let ,init
     ,@body
     (varlet (outlet (curlet))
       ,@(map (lambda (export)
		`(cons ',export ,export))
	      (car end)))
     ,@(cdr end)))
</pre>

<!--
(define-macro (safe-let! init end . body)
  `(with-let (#_inlet (unlet)
	,@(#_map (#_lambda (b) 
		 `(#_cons ',(#_car b) ,(#_cadr b)))
	       init))
     ,@body
     (#_varlet (#_outlet (#_curlet))
	,@(#_map (#_lambda (export)
		 `(#_cons ',export ,export))
	       (#_car end)))
     ,@(#_cdr end)))
-->

</div>

<div class="indented">
<p>Well, almost, darn it.  If the loaded library file sets (via set!) a global value
such as abs, we need to put it back to its original form:
</p>

<pre>
(define (safe-load file)
  (let ((e (with-let (unlet)         ; save the environment before loading
	     (let-&gt;list (curlet)))))
    (<em class=red>load</em> file (curlet))
    (let ((new-e (with-let (unlet)   ; get the environment after loading
		   (let-&gt;list (curlet)))))
      (for-each                       ; see if any built-in functions were stepped on
       (lambda (sym)
	 (unless (assoc (car sym) e)
	   (format () "~S clobbered ~A~%" file (car sym))
	   (apply set! (car sym) (list (cdr sym)))))
       new-e))))

;; say libtest.scm has the line (set! abs odd?)

&gt; (safe-load "libtest.scm")
<em class="gray">"libtest.scm" clobbered abs</em>
&gt; (abs -2)
<em class="gray">2</em>
</pre>

</div>
</blockquote>


<p><b>openlet</b> marks its argument, either an environment, a closure, or a c-object
as open.  I need better terminology here!  An open object is one that the
built-in s7 functions handle specially.  If they encounter one in their
argument list, they look in the object for their own name, and call that
function if it exists.  A bare-bones example:
</p>

<pre class="indented">
&gt; (abs (openlet (inlet 'abs (lambda (x) 47))))
<em class="gray">47</em>
&gt; (define* (f1 (a 1)) (if (real? a) (abs a) ((a 'f1) a)))
<em class="gray">f1</em>
&gt; (f1 :a (openlet (inlet 'f1 (lambda (e) 47))))
<em class="gray">47</em>
</pre>

<p>In CLOS, we'd declare a class and a method, and call make-instance,
and then discover that it wouldn't work anyway.
Here we have, in effect, an anonymous instance of an anonymous class.
I think this is called a "prototype system"; javascript is apparently similar.
A slightly more complex example:
</p>

<pre class="indented">
(let* ((e1 (openlet 
	   (inlet 
	    'x 3
	    '* (lambda args
		 (apply * (if (number? (car args))
		     	      (values (car args) ((cadr args) 'x) (cddr args))
		              (values ((car args) 'x) (cdr args))))))))
       (e2 (copy e1)))
  (set! (e2 'x) 4)
  (* 2 e1 e2)) ; (* 2 3 4) =&gt; 24
</pre>

<p>Perhaps these names would be better: openlet -&gt; with-methods, coverlet -&gt; without-methods,
and openlet? -&gt; methods?.
</p>

<p><b>object-&gt;let</b> returns an environment (more of a dictionary really) that
contains details about its argument.  It
is intended as a debugging aid, underlying a debugger's "inspect" for example. 
</p>

<pre class="indented">
&gt; (let ((iter (make-iterator "1234")))
    (iter)
    (iter)
    (object-&gt;let iter))
<em class="gray">(inlet 'value #&lt;iterator: string&gt; 'type iterator? 'at-end #f 'sequence "1234" 'length 4 'position 2)</em>
</pre>

<p>A c-object (in the sense of s7_new_type), can add its own info to this namespace via an object-&gt;let
method in its local environment.  snd-marks.c has a simple example using a class-wide environment (g_mark_methods),
holding as the value of its 'object-&gt;let field the function s7_mark_to_let.  The latter uses s7_varlet to
add information to the namespace created by <code>(object-&gt;let mark)</code>.
</p>

<br>


<blockquote>

<details>
<summary class="indented">define-class</summary>
<div class="indented">

<p>Here is yet another object system.  Each class and each instance is an environment.
A class might be thought of as a template for instances, but
there's actually no difference between them.  To make an instance of a class, copy it.  To inherit from
another class, concatenate the environments. To access (get or set) a field or a method, use the implicit
indexing syntax with the field or method name.  To evaluate a form in the context of an instance
(CL's with-slots), use with-let. To run through all the fields, use map or for-each.
</p>

<pre>
(define-macro* (define-class class-name inherited-classes (slots ()) (methods ()))
  `(let ((outer-env (outlet (curlet)))
	 (new-methods ())
	 (new-slots ()))
     
     (for-each
      (lambda (class)
	;; each class is a set of nested environments, the innermost (first in the list)
	;;   holds the local slots which are copied each time an instance is created,
	;;   the next holds the class slots (global to all instances, not copied);
	;;   these hold the class name and other such info.  The remaining environments
	;;   hold the methods, with the localmost method first.  So in this loop, we
	;;   are gathering the local slots and all the methods of the inherited
	;;   classes, and will splice them together below as a new class.
	
	(set! new-slots (append (let-&gt;list class) new-slots))
	(do ((e (outlet (outlet class)) (outlet e)))
	    ((or (not (let? e))
		 (eq? e (rootlet))))
	  (set! new-methods (append (let-&gt;list e) new-methods))))
      ,inherited-classes)
     
     (let ((remove-duplicates 
	    (lambda (lst)         ; if multiple local slots with same name, take the localmost
	      (letrec ((rem-dup
			(lambda (lst nlst)
			  (cond ((null? lst) nlst)
				((assq (caar lst) nlst) (rem-dup (cdr lst) nlst))
				(else (rem-dup (cdr lst) (cons (car lst) nlst)))))))
		(reverse (rem-dup lst ()))))))
       (set! new-slots 
	     (remove-duplicates
	      (append (map (lambda (slot)
			     (if (pair? slot)
				 (cons (car slot) (cadr slot))
				 (cons slot #f)))
			   ,slots)                    ; the incoming new slots, #f is the default value
		      new-slots))))                   ; the inherited slots
     
     (set! new-methods 
	   (append (map (lambda (method)
			  (if (pair? method)
			      (cons (car method) (cadr method))
			      (cons method #f)))
			,methods)                     ; the incoming new methods
		   
		   ;; add an object-&gt;string method for this class (this is already a generic function).
		   (list (cons 'object-&gt;string 
			       (lambda* (obj (use-write #t))
				 (if (eq? use-write :readable)    ; write readably
				     (format #f "(make-~A~{ :~A ~W~^~})" 
					     ',class-name 
					     (map (lambda (slot)
						    (values (car slot) (cdr slot)))
						  obj))
				     (format #f "#&lt;~A: ~{~A~^ ~}&gt;" 
					     ',class-name
					     (map (lambda (slot)
						    (list (car slot) (cdr slot)))
						  obj))))))
		   (reverse! new-methods)))                      ; the inherited methods, shadowed automatically
     
     (let ((new-class (openlet
                       (apply sublet                             ; the local slots
			      (sublet                            ; the global slots
				  (apply inlet                   ; the methods
					 (reverse new-methods))
				'class-name ',class-name         ; class-name slot
				'inherited ,inherited-classes
				'inheritors ())                  ; classes that inherit from this class
			      new-slots))))
       
       (varlet outer-env                  
	 ',class-name new-class                                  ; define the class as class-name in the calling environment
	 
	 ;; define class-name? type check
	 (string-&gt;symbol (string-append (symbol-&gt;string ',class-name) "?"))
	 (lambda (obj)
	   (and (let? obj)
		(eq? (obj 'class-name) ',class-name))))
       
       (varlet outer-env
	 ;; define the make-instance function for this class.  
	 ;;   Each slot is a keyword argument to the make function.
	 (string-&gt;symbol (string-append "make-" (symbol-&gt;string ',class-name)))
	 (apply lambda* (map (lambda (slot)
			       (if (pair? slot)
				   (list (car slot) (cdr slot))
				   (list slot #f)))
			     new-slots)
		`((let ((new-obj (copy ,,class-name)))
		    ,@(map (lambda (slot)
			     `(set! (new-obj ',(car slot)) ,(car slot)))
			   new-slots)
		    new-obj))))
       
       ;; save inheritance info for this class for subsequent define-method
       (letrec ((add-inheritor (lambda (class)
				 (for-each add-inheritor (class 'inherited))
				 (if (not (memq new-class (class 'inheritors)))
				     (set! (class 'inheritors) (cons new-class (class 'inheritors)))))))
	 (for-each add-inheritor ,inherited-classes))
       
       ',class-name)))

(define-macro (define-generic name)    ; (define (genfun any) ((any 'genfun) any))
  `(define ,name 
     (lambda args 
       (let ((gf ((car args) ',name))) ; get local definition
	 (if (not (eq? gf ,name))      ; avoid infinite recursion
             (apply gf args)
	     (error "attempt to call generic function wrapper recursively"))))))

(define-macro (define-slot-accessor name slot)
  `(define ,name (dilambda 
		  (lambda (obj) (obj ',slot)) 
		  (lambda (obj val) (set! (obj ',slot) val)))))

(define-macro (define-method name-and-args . body)
  `(let* ((outer-env (outlet (curlet)))
	  (method-name (car ',name-and-args))
	  (method-args (cdr ',name-and-args))
	  (object (caar method-args))
	  (class (symbol-&gt;value (cadar method-args)))
	  (old-method (class method-name))
	  (method (apply lambda* method-args ',body)))
     
     ;; define the method as a normal-looking function
     ;;   s7test.scm has define-method-with-next-method that implements call-next-method here
     ;;   it also has make-instance 
     (varlet outer-env
       method-name (apply lambda* method-args 
			  `(((,object ',method-name)
			     ,@(map (lambda (arg)
				      (if (pair? arg) (car arg) arg))
				    method-args)))))
     
     ;; add the method to the class
     (varlet (outlet (outlet class)) method-name method)
     
     ;; if there are inheritors, add it to them as well, but not if they have a shadowing version
     (for-each
      (lambda (inheritor) 
	(if (not (eq? (inheritor method-name) #&lt;undefined&gt;)) ; defined? goes to the global env
	    (if (eq? (inheritor method-name) old-method)
		(set! (inheritor method-name) method))
	    (varlet (outlet (outlet inheritor)) method-name method)))
      (class 'inheritors))
     
     method-name))

(define (all-methods obj method)
  ;; for arbitrary method combinations: this returns a list of all the methods of a given name
  ;;   in obj's class and the classes it inherits from (see example below)
  (let* ((base-method (obj method))
	 (methods (if (procedure? base-method) (list base-method) ())))
    (for-each 
     (lambda (ancestor)
       (let ((next-method (ancestor method)))
	 (if (and (procedure? next-method)
		  (not (memq next-method methods)))
	     (set! methods (cons next-method methods)))))
     (obj 'inherited))
    (reverse methods)))


&gt; (define-class class-1 () 
       '((a 1) (b 2)) 
       (list (list 'add (lambda (obj) 
                          (with-let obj
                            (+ a b))))))
<em class="gray">class-1</em>
&gt; (define v (make-class-1 :a 32))
<em class="gray">v </em>
&gt; (v 'a)                         ; to set the 'a slot, (set! (v 'a) 0) 
<em class="gray">32</em>
&gt; (object-&gt;string v)             ; built-in functions are all generic
<em class="gray">"#&lt;class-1: (a 32) (b 2)&gt;"</em>       ;   so this uses the method defined in the class definition
&gt; ((v 'add) v)
<em class="gray">34</em>
&gt; (define-generic add)
<em class="gray">add</em>
&gt; (add v)                        ; syntactic sugar for ((v 'add) v)
<em class="gray">34</em>
&gt; (define-slot-accessor slot-a a) ; more sugar!
<em class="gray">slot-a</em>
&gt; (slot-a v)                     ; same as (v 'a), set via (set! (slot-a v) 123)
<em class="gray">32</em>
&gt; (map car v)                    ; map and for-each work with environments
<em class="gray">(a b)</em>                               ;   map cdr would return '(32 2) in this case
&gt; (define-class class-2 (list class-1)
       '((c 3)) 
       (list (list 'multiply (lambda (obj) 
                               (with-let obj 
                                 (* a b c))))))
<em class="gray">class-2</em>
&gt; (define v2 (make-class-2 :a 32))
<em class="gray">v2</em>
&gt; v2                            ; will use object-&gt;string
<em class="gray">"#&lt;class-2: (c 3) (a 32) (b 2)&gt;"</em>
&gt; ((v2 'multiply) v2)
<em class="gray">192</em>
&gt; (add v2)                      ; inherited from class-1
<em class="gray">34</em>
&gt; (define-method (subtract (obj class-1)) (with-let obj (- a b)))
<em class="gray">subtract</em>
&gt; (subtract v2)  ; class-2 inherits from class-1 so it knows about subtract
<em class="gray">30</em>
&gt; (define v1 (make-class-1))
<em class="gray">v1</em>
&gt; (varlet v1      ; change the add method just in this instance
       'add (lambda (obj) 
              (with-let obj
                (+ 1 a (* 2 b)))))
<em class="gray">#&lt;class-1: (a 1) (b 2) (add #&lt;lambda (obj)&gt;)&gt;</em>
&gt; (add v1)
<em class="gray">6</em>
&gt; (add v)                       ; other class-1 instances are not affected
<em class="gray">34</em>
&gt; (define-class class-3 (list class-2) () 
    (list (list 'multiply (lambda (obj num) 
			    (* num 
			       ((class-2 'multiply) obj) ; method combination 
			       (add obj))))))
<em class="gray">class-3</em>
&gt; ((class-3 'multiply) class-3 10)
<em class="gray">180                               ; (* 10 (* 1 2 3) (+ 1 2))</em>
&gt; (define v3 (make-class-3))
<em class="gray">v3</em>
&gt; (all-methods v3 'multiply)
<em class="gray">(#&lt;lambda (obj num)&gt; #&lt;lambda (obj)&gt;)</em>
&gt; (for-each (lambda (p) (format () "~A~%" (procedure-source p))) (all-methods v3 'multiply))
<em class="gray">(lambda (obj num) (* num ((class-2 'multiply) obj) (add obj)))</em>
<em class="gray">(lambda (obj) (with-let obj (* a b c)))</em>
</pre>

<p>with-let (used briefly above) provides an even more striking simplification of syntax
than implicit indexing or multidimensional vectors, and it is faster as well!
See Snd's generators.scm for many examples.  
</p>

<!--
at class definition time:
    (list 'mac (symbol->value (define-macro (mac a) `(+ ,a 1))))
-->
</div>
</details>


<details>
<summary class="indented">more examples</summary>
<div class="indented">
<p>Implicit indexing of a local environment
does not search the global environment.  Since unlet extends the
current environment chain, it is considered a local environment:
</p>

<pre class="indented">
&gt; ((rootlet) 'abs)
<em class="gray">abs</em>
&gt; (let () ((curlet) 'abs))
<em class="gray">#&lt;undefined&gt;</em>
&gt; ((unlet) 'abs)
<em class="gray">#&lt;undefined&gt;</em>
</pre>

</div>


<div class="indented">
<pre>
(define-macro (value-&gt;symbol expr)
  `(let ((val ,expr)
	 (e1 (curlet)))
     (call-with-exit
      (lambda (return)
	(do ((e e1 (outlet e))) ()
	  (for-each 
	   (lambda (slot)
	     (if (equal? val (cdr slot))
		 (return (car slot))))
	   e)
	  (if (eq? e (rootlet))
	      (return #f)))))))

&gt; (let ((a 1) (b "hi")) 
    (value-&gt;symbol "hi"))
<em class="gray">b</em>
</pre>
</div>


<div class="indented">
<p>openlet alerts the rest of s7 that the environment has methods.
</p>

<pre>
(begin
  (define fvector? #f)
  (define make-fvector #f)
  (let ((type (gensym))
	(-&gt;float (lambda (x)
		   (if (real? x)
		       (* x 1.0)
		       (error 'wrong-type-arg "fvector new value is not a real: ~A" x)))))
    (set! make-fvector
	  (lambda* (len (init 0.0)) 
	    (<em class=red>openlet</em>
	     (inlet :v (make-vector len (-&gt;float init))
	            :type type
	   	    :length (lambda (f) len)
		    :object-&gt;string (lambda (f . args) "#&lt;fvector&gt;")
		    :let-set! (lambda (fv i val) (#_vector-set! (fv 'v) i (-&gt;float val)))
		    :let-ref (lambda (fv i) (#_vector-ref (fv 'v) i))))))
    (set! fvector? (lambda (p)
		     (and (let? p)
			  (eq? (p 'type) type))))))
  
&gt; (define fv (make-fvector 32))
<em class="gray">fv</em>
&gt; fv
<em class="gray">#&lt;fvector&gt;</em>
&gt; (length fv)
<em class="gray">32</em>
&gt; (set! (fv 0) 123)
<em class="gray">123.0</em>
&gt; (fv 0)
<em class="gray">123.0</em>
</pre>

</div>


<div class="indented">

<p>I can't resist adding at least some of a quaternion implementation
(finally "number?" is not the same
as "complex?"!):
</p>

<pre>
(define-class quaternion () 
  '((r 0) (i 0) (j 0) (k 0))
  (list (list 'number? (lambda (obj) #t))
	(list 'complex? (lambda (obj) #f))
	(list 'real? (lambda (obj) #f))
	(list 'integer? (lambda (obj) #f))
	(list 'rational? (lambda (obj) #f))

	(list '+ (lambda orig-args
		   (let add ((r ()) (i ()) (j ()) (k ()) (args orig-args))
		     (if (null? args)
			 (<em class=red>make-quaternion</em> 
                           (apply + r) (apply + i) (apply + j) (apply + k)) 
			 (let ((n (car args)))
			   (cond
			    ((real? n)
			     (add (cons n r) i j k (cdr args)))
			    ((complex? n)
			     (add (cons (real-part n) r) (cons (imag-part n) i) j k (cdr args)))
			    ((<em class=red>quaternion?</em> n)
			       (add (cons (n 'r) r) (cons (n 'i) i) 
                                    (cons (n 'j) j) (cons (n 'k) k) 
                                    (cdr args)))
                            ((not (<em class=red>openlet?</em> n))   ; maybe we'll add octonions later!
                             (error 'wrong-type-arg "+ argument ~A is not a number" n))
                            ((eq? n (car orig-args))
                             (error 'missing-method "+ can't handle these arguments: ~A" args))
                            (else
			      (apply (n '+) 
                                     (make-quaternion 
                                       (apply + r) (apply + i) (apply + j) (apply + k)) 
                                     (cdr args)))))))))))

&gt; (let ((q1 (make-quaternion 1.0 1.0 0.0 0.0)))
    (+ 1 q1 2.5+i))
<em class="gray">#&lt;quaternion: (r 4.5) (i 2.0) (j 0.0) (k 0.0)&gt;</em>
</pre>

</div>
</details>

<div class="indented">
<p>If an s7 function ignores the type of an argument, as in cons or vector for example,
then that argument won't be treated as having any methods.
</p>

<p>
Since outlet is settable, there are two ways an environment can
become circular.  One is to include the current environment as the value of one of its variables.
The other is: <code>(let () (set! (outlet (curlet)) (curlet)))</code>.
</p>

<p>If you want to hide an environment's fields from any part of s7 that does not
know the field names in advance,
</p>
<pre class="indented">
(openlet  ; make it appear to be empty to the rest of s7
  (inlet 'object-&gt;string  (lambda args "#&lt;let&gt;")
         'map             (lambda args ())
         'for-each        (lambda args #&lt;unspecified&gt;)
  	 'let-&gt;list       (lambda args ())
         'length          (lambda args 0)
	 'copy            (lambda args (inlet))
 	 'open #t
	 'coverlet        (lambda (e) (set! (e 'open) #f) e)
	 'openlet         (lambda (e) (set! (e 'open) #t) e)
	 'openlet?        (lambda (e) (e 'open))
         ;; your secret data here
         ))
</pre>
<p>(There are still at least two ways to tell that something is fishy).
</p>
<!-- add a field and it disappears, or sublet and read back -->
</div>
</blockquote>


<div class="header" id="multiplevalues"><h4>multiple-values</h4></div>

<p>
In s7, multiple values are spliced directly into the caller's argument list.
</p>

<pre class="indented">
&gt; (+ (values 1 2 3) 4)
<em class="gray">10</em>
&gt; (string-ref ((lambda () (values "abcd" 2))))
<em class="gray">#\c</em>
&gt; ((lambda (a b) (+ a b)) ((lambda () (values 1 2))))
<em class="gray">3</em>
&gt; (+ (call/cc (lambda (ret) (ret 1 2 3))) 4) ; call/cc has an implicit "values"
<em class="gray">10</em>
&gt; ((lambda* ((a 1) (b 2)) (list a b)) (values :a 3))
<em class="gray">(3 2)</em>

(define-macro (call-with-values producer consumer) 
  `(,consumer (,producer)))

(define-macro (multiple-value-bind vars expr . body)
  `((lambda ,vars ,@body) ,expr))

(define-macro (define-values vars expression)
  `(if (not (null? ',vars))
       (varlet (curlet) ((lambda ,vars (curlet)) ,expression))))

(define (curry function . args)
  (if (null? args)
      function
      (lambda more-args
        (if (null? more-args)
            (apply function args)
            (function (apply values args) (apply values more-args))))))
</pre>


<blockquote>

<div class="indented">

<p>multiple-values are useful in a several situations.  For example,
<code>(if test (+ a b c) (+ a b d e))</code> can be written
<code>(+ a b (if test c (values d e)))</code>. 
There are a few special uses of multiple-values.
First, you can use the values function to return any number of values, including 0,
from map's function application:
</p>

<pre class="indented">
&gt; (map (lambda (x) (if (odd? x) (values x (* x 20)) (values))) (list 1 2 3))
<em class="gray">(1 20 3 60)</em>
&gt; (map values (list 1 2 3) (list 4 5 6))
<em class="gray">(1 4 2 5 3 6)</em>

(define (remove-if func lst) 
  (map (lambda (x) (if (func x) (values) x)) lst))

(define (pick-mappings func lst)          
  (map (lambda (x) (or (func x) (values))) lst))

(define (shuffle . args) 
  (apply map values args))

&gt; (shuffle '(1 2 3) #(4 5 6) '(7 8 9))
<em class="gray">(1 4 7 2 5 8 3 6 9)</em>

(define (concatenate . args)
  (apply append (map (lambda (arg) (map values arg)) args)))
</pre>

<p>Second, a macro can return multiple values; these are evaluated and spliced,
exactly like a normal macro,
so you can use <code>(values '(define a 1) '(define b 2))</code> to
splice multiple definitions at the macro invocation point.  
If an expansion returns (values), nothing is spliced in.  This is
mostly useful in <a href="#readercond">reader-cond</a> and the #; reader.
</p>

<pre class="indented">
&gt; (define-expansion (comment str) (values))
<em class="gray">comment</em>
&gt; (+ 1 (comment "one") 2 (comment "two"))
<em class="gray">3</em>
</pre>

<p>At the top-level (in the REPL), since there's nothing to splice into, you simply get your values back:
</p>

<pre class="indented">
&gt; (values 1 (list 1 2) (+ 3 4 5))
<em class="gray">(values 1 (1 2) 12)</em>
</pre>

<p>But this printout is just trying to be informative.  There is no multiple-values object
in s7.  You can't <code>(set! x (values 1 2))</code>, for example.  The values function
tells s7 that its arguments should be handled in a special way, and the multiple-value indication goes away
as soon as the arguments are spliced into some caller's arguments.  
</p>

<p>Internally, s7 uses <code>(apply values ...)</code> to implement unquote splicing (",@") in quasiquote.
<!--
Is <code>(apply apply func arglist)</code> the same as <code>(apply func (apply values arglist))</code>,
or (leaving aside <code>'(()))</code>, <code>(func (apply values (apply values arglist)))</code>?
-->
</p>
</div>


<div class="indented">

<p>Since set! does not evaluate its first argument, and
there is no setter for "values", <code>(set! (values x) ...)</code> is not
the same as <code>(set! x ...)</code>.  <code>(string-set! (values string) ...)</code>
works because string-set! does evaluate its first argument.  <code>((values + 1 2) (values 3 4) 5)</code>
is 15, as anyone would expect.
</p>

</div>

<!--
<details>
<summary class="indented">more examples</summary>

<pre>
(define (flatten lst) 
  (map values (list (let flatten-1 ((lst lst))
                      (cond ((null? lst) (values))
                            ((not (pair? lst)) lst)
                            (else (values (flatten-1 (car lst)) 
                                          (flatten-1 (cdr lst)))))))))
</pre>

<div class="indented">
</details>
-->


<details>
<summary class="indented">more on values</summary>
<div class="indented">

<p>In some Schemes, values behaves like CL's prog1:
</p>

<pre class="indented">
(not s7)&gt; (let ((x 1)) (cond ((values #f (set! x 2) #t) 3) (#t x)))
<em class="gray">2</em>
(not s7)&gt; (if (values #f #t) 1 2)
<em class="gray">2</em>
</pre>

<p>But in s7 we're trying to implement real multiple values (else why have them at all?).
There are many ways we could interpret <code>(cond ((values ...))...)</code> and
<code>(cond ((values...) =&gt; func))</code>, but surely
equivalent uses of "cond" and "if" should give the same result.
Currently in s7, where a test is in progress, only <code>(values #f)</code> is the same as #f.
</p>

<pre class="indented">
&gt; (if (values #f #f) 1 2)            ; (values #f #f) is not #f
<em class="gray">1</em>
&gt; (cond ((values #f #f) 1) (#t 2))
<em class="gray">1</em>
;;; but if we interpreted this as splicing in the values, we get an inconsistency:
&gt; (cond (#f #f 1) (#t 2))
<em class="gray">2</em>
&gt; (if (values #f) 1 2)
<em class="gray">2</em>
&gt; (cond ((values #f) 1) (#t 2))
<em class="gray">2</em>
&gt; (if (values) 1 2)
<em class="gray">1</em>
&gt; (cond ((values) 1) (#t 2))
<em class="gray">1</em>
;;; this is consistent with (cond (1) (#t 2))
</pre>

<p>
So "if" and "cond" agree, but it requires that in one case the "values"
behavior is slightly weird.  <code>(or (values #f #f))</code> is #f, but that isn't inconsistent because
"or" is not testing anything.
We might choose to say that <code>(if (values #f #f)...)</code>
is an error, but that would be hasty &mdash;
our troubles have only begun.  First, "cond" can omit the expressions that follow the test, unlike "if":
</p>

<pre class="indented">
&gt; (cond (3))
<em class="gray">3</em>
</pre>

<p>and even trickier, "cond" can pass the test value to a function:
</p>

<pre class="indented">
&gt; (cond (3 =&gt; +))
<em class="gray">3</em>
</pre>

<p>The various standards agree that in the "=&gt;" case, the "fed to" function
receives one argument, so
</p>

<pre class="indented">
(not s7)&gt; (cond ((values 1 2) =&gt; +))
<em class="gray">1</em>
</pre>

<p>If we were following the "splice immediately" model, this would be <code>(cond (1 2 =&gt; +))</code>
which is an error in some Schemes.
So something has to give.  My druthers is to make "values" work as consistently as possible, and hope
that the one odd corner will not trip anyone.  From that point of view, the "one arg" standard
looks like a wasted opportunity.
s7 handles it this way:
</p>

<pre class="indented">
&gt; (+ 1 (cond ((values 2 3))) 4)   ; trailing values are not ignored
<em class="gray">10</em>
&gt; (cond ((values 1 2 3) =&gt; +))
<em class="gray">6</em>
</pre>

<p>Of course, it is unproblematic that the expression can itself involve multiple values:
</p>

<pre class="indented">
&gt; (+ (cond (#t (values 1 2))))
<em class="gray">3</em>
</pre>

<p>Now, what have I missed?
</p>
</div>
</details>

</blockquote>

<!-- one place where multiple values might cause surprise:
    :(define* (a1 (a2 (values 1 2))) a2)
    :(a1)
    (values 1 2)
    :(define (a1 a2) a2)
    :(a1 (values 1 2))
    ;(values 1 2): too many arguments: (values 1 2)
-->


<div class="header" id="callwithexit1"><h4>call-with-exit, with-baffle and continuation?</h4></div>


<p><b><em class=def id="callwithexit">call-with-exit</em></b> is call/cc without the ability to jump back into the original context,
similar to "return" in C.  This 
is cleaner than call/cc, and much faster. 
</p>

<pre class="indented">
(define-macro (block . body) 
  ;; borrowed loosely from CL &mdash; predefine "return" as an escape
  `(<em class=red>call-with-exit</em> (lambda (return) ,@body)))

(define-macro (while test . body)      ; while loop with predefined break and continue
  `(<em class=red>call-with-exit</em>
    (lambda (break) 
      (let continue ()
	(if (let () ,test)
	    (begin 
	      (let () ,@body)
	      (continue))
	    (break))))))

(define-macro (switch selector . clauses) ; C-style case (branches fall through unless break called)
  `(<em class=red>call-with-exit</em>
    (lambda (break)
      (case ,selector
	,@(do ((clause clauses (cdr clause))
	       (new-clauses ()))
	      ((null? clause) (reverse new-clauses))
	    (set! new-clauses (cons `(,(caar clause) 
				      ,@(cdar clause)
				      ,@(map (lambda (nc)
					       (apply values (cdr nc))) ; doubly spliced!
					     (if (pair? clause) (cdr clause) ())))
				    new-clauses)))))))

(define (and-for-each func . args)
  ;; apply func to the first member of each arg, stopping if it returns #f
  (<em class=red>call-with-exit</em>
   (lambda (quit)
     (apply for-each (lambda arglist
		       (if (not (apply func arglist))
			   (quit #&lt;unspecified&gt;))) 
	    args))))

(define (find-if f . args)  ; generic position-if is very similar
  (<em class=red>call-with-exit</em>
   (lambda (return) 
     (apply for-each (lambda main-args 
		       (if (apply f main-args) 
			   (apply return main-args)))
	    args))))

&gt; (find-if even? #(1 3 5 2))
<em class="gray">2</em>
&gt; (* (find-if > #(1 3 5 2) '(2 2 2 3)))
<em class="gray">6</em>
</pre>

<p>
The call-with-exit function's argument (the "continuation") is only valid
within the call-with-exit function.  In call/cc, you can save it, then call it later
to jump back, but if you try that with call-with-exit (from outside the call-with-exit function's body), you'll get an error.
This is similar to trying to read from a closed input port. 
</p>


<p>The other side, so to speak, of call-with-exit, is <em class=def id="withbaffle">with-baffle</em>.
Both limit the scope of a continuation.
Sometimes we need a normal call/cc, but want to make sure it is active
only within a given block of code.  
Normally, if a continuation gets away, there's no telling when it might wreak havoc on us.
Scheme code with call/cc becomes unpredictable, undebuggable, and completely
unmaintainable.  
with-baffle blocks all that &mdash; no continuation can jump into its body:
</p>

<pre class="indented">
(let ((what's-for-breakfast ())
      (bad-dog 'fido))        ; bad-dog wonders what's for breakfast?
  (<em class=red>with-baffle</em>                ; the syntax is (with-baffle . body)         
   (set! what's-for-breakfast
	 (call/cc
	  (lambda (biscuit?)
	    (set! bad-dog biscuit?) ; bad-dog smells a biscuit!
	    'biscuit!))))
  (if (eq? what's-for-breakfast 'biscuit!) 
      (bad-dog 'biscuit!))     ; now, outside the baffled block, bad-dog wants that biscuit!
  what's-for-breakfast)        ;   but s7 says "No!": baffled! ("continuation can't jump into with-baffle")
</pre>

<br>
<p><em class=def id="continuationp">continuation?</em> returns #t if its argument is a continuation,
as opposed to a normal procedure.  I don't know why Scheme hasn't had this function from
the very beginning, but it's needed if you want to write a continuable error
handler.  Here is a sketch of the situation:
</p>

<pre class="indented">
(let ()
  (catch #t
	 (lambda ()
	   (let ((res (call/cc 
                        (lambda (ok) 
			  (error 'cerror "an error" ok)))))
	     (display res) (newline)))
	 (lambda args
	   (if (and (eq? (car args) 'cerror)
		    (<em class=red>continuation?</em> (cadadr args)))
	       (begin
		 (display "continuing...")
		 ((cadadr args) 2)))
	   (display "oops"))))

continuing...2
</pre>

<p>In a more general case, the error handler is separate from the
catch body, and needs a way to distinguish a real continuation
from a simple procedure.  
</p>

<pre class="indented">
(define (continuable-error . args)
  (call/cc 
   (lambda (continue)
     (apply error args))))

(define (continue-from-error)
  (if (<em class=red>continuation?</em> ((owlet) 'continue)) ; might be #&lt;undefined&gt; or a function as in the while macro
      (((owlet) 'continue))
      'bummer))
</pre>

<!--
(define-macro (call-with-exit func)
  (let ((tag (caadr func)))
    `(catch ',tag
       (lambda ()
	 (define-macro (,tag . body) 
	   `(throw ',',tag ,@body))
	 ,@(cddr func))
       (lambda (type info)
	 (car info)))))
-->


<div class="header" id="format1"><h4>format, object-&gt;string</h4></div>

<p>object-&gt;string returns the string representation of its argument.  Its optional second argument
can be #f (use display), #t (the default, use write), or :readable.  In the latter case, object-&gt;string
tries to produce a string that can be evaluated via eval-string to return an object equal to the
original.
</p>

<pre class="indented">
&gt; (object-&gt;string "hiho")
<em class="gray">"\"hiho\""</em>
&gt; (format #f "~S" "hiho")
<em class="gray">"\"hiho\""</em>
</pre>

<br>
<p>s7's <em class=def id="format">format</em> function is very close to that in srfi-48.
</p>

<pre class="indented">
&gt; (format #f "~A ~D ~F" 'hi 123 3.14)
<em class="gray">"hi 123 3.140000"</em>
</pre>


<p>The format directives (tilde chars) are:</p>
<pre class="indented">
~%        insert newline
~&amp;        insert newline if preceding char was not newline
~~        insert tilde
~\n       (tilde followed by newline): trim white space
~{        begin iteration (take arguments from a list, string, vector, or any other applicable object)
~}        end iteration
~^ ~|     jump out of iteration
~*        ignore the current argument
~C        print character (numeric argument = how many times to print it)
~P        insert 's' if current argument is not 1 or 1.0 (use ~@P for "ies" or "y")
~A        object-&gt;string as in display
~S        object-&gt;string as in write
~B        number-&gt;string in base 2
~O        number-&gt;string in base 8
~D        number-&gt;string in base 10
~X        number-&gt;string in base 16
~E        float to string, (format #f "~E" 100.1) -&gt; "1.001000e+02", (%e in C)
~F        float to string, (format #f "~F" 100.1) -&gt; "100.100000",   (%f in C)
~G        float to string, (format #f "~G" 100.1) -&gt; "100.1",        (%g in C)
~T        insert spaces (padding)
~N        get numeric argument from argument list (similar to ~V in CL)
~W        object-&gt;string with :readable (write readably; s7 is the intended reader)
</pre>

<p>The eight directives before ~W take the usual numeric arguments to specify field width and precision.
These can also be ~N or ~n in which case the numeric argument is read from the list of arguments:
</p>
<pre class="indented">
(format #f "~ND" 20 1234) ; =&gt; (format "~20D" 1234)
<em class="gray">"                1234"</em>
</pre>

<p>
<code>(format #f ...)</code> simply returns the formatted string; <code>(format #t ...)</code>
also sends the string to the current-output-port.  <code>(format () ...)</code> sends the output to 
the current-output-port without returning the string (this mimics the other IO routines
such as display and newline).  Other built-in port choices are *stdout* and *stderr*.
</p>


<blockquote>

<div class="indented">

<p>Floats can occur in any base, so:
</p>

<pre class="indented">
&gt; #xf.c
<em class="gray">15.75</em>
</pre>

<p>This also affects format.  In most Schemes, <code>(format #f "~X" 1.25)</code> is
an error.  In CL, it is equivalent to using ~A which is perverse.  But 
</p>

<pre class="indented">
&gt; (number-&gt;string 1.25 16)
<em class="gray">"1.4"</em>
</pre>

<p>and there's no obvious way to get the same effect from format unless we accept
floats in the "~X" case.  So in s7, 
</p>

<pre class="indented">
&gt; (format #f "~X" 21)
<em class="gray">"15"</em>
&gt; (format #f "~X" 1.25)
<em class="gray">"1.4"</em>
&gt; (format #f "~X" 1.25+i)
<em class="gray">"1.4+1.0i"</em>
&gt; (format #f "~X" 21/4)
<em class="gray">"15/4"</em>
</pre>

<p>That is, the output choice matches the argument.  A case that came up in the Guile mailing lists is:
<code>(format #f "~F" 1/3)</code>.  s7 currently returns "1/3", but Clisp returns "0.33333334".
</p>
<br>
<p>The curly bracket directive applies to anything you can map over, not just lists:
</p>

<pre class="indented">
&gt; (format #f "~{~C~^ ~}" "hiho")
<em class="gray">"h i h o"</em>
&gt; (format #f "~{~{~C~^ ~}~^...~}" (list "hiho" "test"))
<em class="gray">"h i h o...t e s t"</em>
&gt; (with-input-from-string (format #f "(~{~C~^ ~})" (format #f "~B" 1367)) read) ; integer-&gt;list
<em class="gray">(1 0 1 0 1 0 1 0 1 1 1)</em>
</pre>
<br>

<p>Since any sequence can be passed to ~{~}, we need a way to truncate output and represent
the rest of the sequence with "...", but ~^ only stops at the end of the sequence.  ~|
is like ~^ but it also stops after it handles (*s7* 'print-length) elements and prints
"...".  So, <code>(format #f "~{~A~| ~}" #(0 1 2 3 4))</code> returns "0 1 2 ..."
if (*s7* 'print-length) is 3.
</p>

</div>
</blockquote>


<blockquote>

<div class="indented">

<p>I added object-&gt;string to s7 before deciding to include format.  format excites a
vague disquiet &mdash; why do we need this ancient unlispy thing?
We can almost replace it with:
</p>

<pre class="indented">
(define (objects-&gt;string . objects)
  (apply string-append (map (lambda (obj) (object-&gt;string obj #f)) objects)))
</pre>

<p>But how to handle lists (~{...~} in format), or columnized output (~T)?
I wonder whether formatted string output still matters outside a REPL.  Even in that context,
a modern GUI leaves formatting decisions to a text or table widget.
</p>

<pre class="indented">
(define-macro (string-&gt;objects str . objs)
  `(with-input-from-string ,str
     (lambda ()
       ,@(map (lambda (obj)
		`(set! ,obj (eval (read))))
	      objs))))
</pre>


<!--

:(objects->string "int: " 32 ", string: " "hi")
"int: 32, string: hi"

(define (cycle->string . objs)
  (call-with-exit
   (lambda (return)
     (for-each
      (lambda (obj)
	(if (pair? obj)
	    (return
	     (string-append 
	      (apply objects->string 
		     (map (lambda (obj)
			    (if (pair? obj)
				(car obj)
				obj))
			  objs))
	      (apply cycle->string 
		     (map (lambda (obj)
			    (if (pair? obj)
				(cdr obj)
				obj))
			  objs))))))
      objs)
     "")))

;;; (cycle->string ": " (list 1 2 3) " |")
:(objects->string "int: " 32 ", list with spaces: (" (cycle->string (list 1 2 3) " ") "), string: " "hi")
"int: 32, list with spaces: (1 2 3 ), string: hi"

:(let ((x 0) (y 0)) (string->objects "1 2" x y) (list x y))
(1 2)

-->

</div>

</blockquote>


<div class="header" id="hooks"><h4>hooks</h4></div>

<pre class="indented">
(<em class=def id="makehook">make-hook</em> . fields)           ; make a new hook
(<em class=def id="hookfunctions">hook-functions</em> hook)          ; the hook's list of 'body' functions
</pre>

<p>A hook is a function created by make-hook, and called (normally from C) when something interesting happens.
In GUI toolkits hooks are called callback-lists, in CL conditions,
in other contexts watchpoints or signals.  s7 itself has several
hooks: <a href="#errorhook">*error-hook*</a>, <a href="#readerrorhook">*read-error-hook*</a>,
<a href="#unboundvariablehook">*unbound-variable-hook*</a>, *missing-close-paren-hook*,
and <a href="#loadhook">*load-hook*</a>.
make-hook is:
</p>

<pre class="indented">
(define (make-hook . args)
  (let ((body ()))
    (apply lambda* args
      '(let ((result #&lt;unspecified&gt;))
         (let ((e (curlet)))
	   (for-each (lambda (f) (f e)) body) 
           result))
       ())))
</pre>

<p>So the result of calling make-hook is a function (the lambda* that is applied to args above) that
contains a list of functions, 'body.  
Each function in that list takes one argument, the hook.
Each time the hook itself is called, each of the body functions is called, and the value of 'result is returned.
That variable, and each of the hook's arguments are accessible to the hook's internal
functions by going through the environment passed to the internal functions.  This is a bit circuitous; 
here's a sketch:
</p>

<pre class="indented">
&gt; (define h (make-hook '(a 32) 'b))     ; h is a function: (lambda* ((a 32) b) ...)
<em class="gray">h</em>
&gt; (set! (hook-functions h)              ; this sets ((funclet h) 'body)
           (list (lambda (hook) 	; each hook internal function takes one argument, the environment
                   (set! (hook 'result) ; this is the "result" variable above 
                         (format #f "a: ~S, b: ~S" (hook 'a) (hook 'b))))))
<em class="gray">(#&lt;lambda (hook)&gt;)</em>
&gt; (h 1 2)                               ; this calls the hook's internal functions (just one in this case)
<em class="gray">"a: 1, b: 2"                            ; we set "result" to this string, so it is returned as the hook application result</em>
&gt; (h)
<em class="gray">"a: 32, b: #f"</em>
</pre>

<p>In C, to make a hook:
</p>

<pre class="indented">
hook = s7_eval_c_string("(make-hook '(a 32) 'b)");
s7_gc_protect(s7, hook);
</pre>

<p>And call it:
</p>

<pre class="indented">
result = s7_call(s7, hook, s7_list(s7, 2, s7_make_integer(s7, 1), s7_make_integer(s7, 2)));
</pre>

<div class="indented">
<pre>
(define-macro (hook . body)  ; return a new hook with "body" as its body, setting "result"
  `(let ((h (make-hook)))
     (set! (hook-functions h) (list (lambda (h) (set! (h 'result) (begin ,@body)))))
     h))
</pre>
</div>


<div class="header" id="procedureinfo"><h4>procedure info</h4></div>


<pre class="indented">
(<em class=def id="proceduresource">procedure-source</em> proc)
(<em class=def id="proceduredocumentation">procedure-documentation</em> proc)
(<em class=def id="proceduresignature">procedure-signature</em> proc)
(<em class=def id="proceduresetter">procedure-setter</em> proc)
(funclet proc)

(<em class=def id="arity">arity</em> obj)
(<em class=def id="aritablep">aritable?</em> obj num-args)
</pre>

<p>
procedure-setter returns or sets the set function associated with a procedure (set-car! with car, for example).
funclet returns
a procedure's environment.  
procedure-source returns the procedure source (a list). 
</p>

<pre class="indented">
(define (procedure-arglist f) (cadr (procedure-source f)))
</pre>

<p>
procedure-documentation returns the documentation string associated with a procedure.  This used to be 
the initial string in the function's body (as in CL), but now it is the value of the 'documentation variable, if any,
in the procedure's local environment.  That is, <code>(define (f a) "doc string" a)</code> is now
<code>(define f (let ((documentation "doc string")) (lambda (a) a)))</code>.  This change gets the 
string out of the body (where it can slow down evaluation of the function to no purpose), and
makes it possible to extend the function information arbitrarily. 
</p>

<p>
arity takes any object and returns either #f if it is not applicable,
or a cons containing the minimum and maximum number of arguments acceptable.  If the maximum reported
is a really big number, that means any number of arguments is ok.
aritable? takes two arguments, an object and an integer, and returns #t if the object can be
applied to that many arguments. 
</p>

<pre class="indented">
&gt; (define add-2 (let ((documentation "add-2 adds its 2 args")) (lambda* (a (b 32)) (+ a b))))
<em class="gray">add-2</em>
&gt; (procedure-documentation add-2)
<em class="gray">"add-2 adds its 2 args"</em>
&gt; (procedure-source add-2)
<em class="gray">(lambda* (a (b 32)) (+ a b))</em>
&gt; (arity add-2)
<em class="gray">(0 . 2)</em>
</pre>

<p>
procedure-signature is a list describing the argument types and returned value type
of the function.  The first entry in the list is the return type, and the rest are
argument types.  #t means any type is possible, and 'values means the function returns multiple values.
</p>

<pre class="indented">
&gt; (procedure-signature round)
<em class="gray">(integer? real?)</em> ; round takes a real argument, returns an integer
&gt; (procedure-signature vector-ref)
<em class="gray">(#t vector? . #1=(integer? . #1#))</em> ; trailing args are all integers (indices)
</pre>

<p>If an entry is a list, any of the listed types can occur:
</p>

<pre class="indented">
&gt; (procedure-signature char-position)
<em class="gray">((boolean? integer?) (char? string?) string? integer?)</em>
</pre>

<p>which says that the first argument to char-position can be a string or a character,
and the return type can be either boolean or an integer.
If the function is defined in scheme, its signature is the value of the 'signature variable
in its closure:
</p>

<pre class="indented">
&gt; (define f1 (let ((documentation "helpful info") 
                   (signature '(boolean? real?))) 
                (lambda (x) 
                  (positive? x))))
<em class="gray">f1</em>
&gt; (procedure-documentation f1)
<em class="gray">"helpful info"</em>
&gt; (procedure-signature f1)
<em class="gray">(boolean? real?)</em>
</pre>

<p>We could do the same thing using methods:
</p>

<pre class="indented">
&gt; (define f1 (let ((procedure-documentation (lambda (f) "helpful info"))
                    (procedure-signature (lambda (f) '(boolean? real?))))
                (<em class=red>openlet</em>
                  (lambda (x) 
                    (positive? x)))))
&gt; (procedure-documentation f1)
<em class="gray">"helpful info"</em>
&gt; (procedure-signature f1)
<em class="gray">(boolean? real?)</em>
</pre>

<p>openlet alerts s7 that f1 has methods.
</p>
<blockquote>
<br>

<details>
<summary class="indented">examples</summary>
<div class="indented">

<p>procedure-source returns the actual function source.  If you
call the function, the optimizer changes the source to suit itself,
so if you want to walk over the function body or change it in some way,
make a clean copy of
the procedure-source first (using, for example, copy-tree in stuff.scm).
</p>

<pre>
(define-bacro (procedure-annotate proc) ; use "bacro" so we can annotate local functions
  (let ((orig (<em class=red>procedure-source</em> proc))) ; this assumes we haven't called "proc" yet

    (define (proc-walk source)
      (cond ((not (pair? source))
	     source)

	    ((not (memq (car source) '(let let*))) ; if let or let*, show local variables
	     (cons (proc-walk (car source))
		   (proc-walk (cdr source))))

	    ((symbol? (cadr source))               ; named let?
	     (append               ; (let name vars . body) -&gt; (let name vars print-vars . body)
	       (list (car source)
		     (cadr source)
		     (caddr source)
		     `(format () "    (let ~A (~{~A~^ ~}) ...)~%" ,(cadr source) (curlet)))
	       (cdddr source)))
         
            (else                  ; (let(*) vars . body) -&gt; (let vars print-vars . body)
	      (append 
	        (list (car source)
		      (cadr source)
		      `(format () "    (~A (~{~A~^ ~}) ...)~%" ,(car source) (curlet)))
	        (cddr source)))))
    
    (let* ((new-body (proc-walk orig))
	   (result (gensym))
	   (new-source 
	    `(lambda ,(cadr orig)
	       (let ((,result #&lt;undefined&gt;))
		 (dynamic-wind
		     (lambda ()       ; upon entry, show procedure name and args
		       (format () "(~A~{ ~A~})~%" 
                               ',proc 
			       (outlet (outlet (curlet)))))
		     (lambda ()
		       (set! ,result (,new-body ,@(cadr orig))))
		     (lambda ()       ; at exit, show result
		       (if (eq? ,result #&lt;undefined&gt;)
			   (format () "  ~A returns early~%" ',proc)
			   (format () "  ~A returns ~A~%" ',proc ,result))))))))

      `(define ,proc ,new-source))))

			 
&gt; (define (hi a) (let ((b 12)) (+ b a)))
<em class="gray">hi</em>
&gt; (procedure-annotate hi)
<em class="gray">#&lt;lambda (a)&gt;</em>
&gt; (let ((x 32)) (+ 1 (hi x)))
<em class="gray">45</em>
;; printing: 
(hi (a . 32))
    (let ((b . 12)) ...)
  hi returns 44
</pre>

<p>But maybe something less invasive is better. Here's a version of let that prints
its bindings (this is borrowed from "nuntius" at reddit lisp):
</p>

<pre>
(define-macro (print-let bindings . body)
  (let ((temp-symbol (gensym)))
    `(let ,(map (lambda (var/val)
		  `(,(car var/val) 
		    (let ((,temp-symbol ,(cadr var/val))) 
		      (format () ";~S: ~S -&gt; ~S~%" 
			      ',(car var/val) 
			      ',(cadr var/val) 
			      ,temp-symbol)
		      ,temp-symbol)))
		bindings)
       ,@body)))

;; or simpler:

(define-macro (print-let bindings . body)
  `(let ,bindings 
     (format () "~{;~A~%~}" (curlet))
     ,@body))	      
</pre>

</div>


<div class="indented">

<p>A minor footnote: there are cases in s7 where aritable? can't tell whether
a given number of arguments can be applied to an object.  For example,
<code>((list 1 (list 2 3)) 0 1)</code> is an error, but
<code>((list 1 (list 2 3)) 1 1)</code> is 3.  So 
<code>(aritable? (list 1 (list 2 3)) 2)</code> depdends on
what actual arguments you pass.  In these cases, aritable? returns <code>#f</code>.
</p>

</div>
</details>

<div class="indented">

<pre>
(define (for-each-subset func args)
  ;; form each subset of args, apply func to the subsets that fit its arity
  (let subset ((source args)
               (dest ())
               (len 0))
    (if (null? source)
        (if (<em class=red>aritable?</em> func len)   ; does this subset fit?
            (apply func dest))
        (begin
          (subset (cdr source) (cons (car source) dest) (+ len 1))
          (subset (cdr source) dest len)))))
</pre>
</div>
</blockquote>


<div class="header" id="evalstring"><h4>eval</h4></div>

<p>
<b>eval</b> evaluates its argument, a list representing a piece of code.  It takes an optional
second argument, the environment in which the evaluation should take place.  <b>eval-string</b>
is similar, but its argument is a string.
</p>

<pre class="indented">
&gt; (eval '(+ 1 2))
<em class="gray">3</em>
&gt; (eval-string "(+ 1 2)")
<em class="gray">3</em>
</pre>

<p>Leaving aside a few special cases, eval-string could be defined:
</p>
<pre class="indented">
(define-macro* (eval-string x e)
  `(eval (with-input-from-string ,x read) (or ,e (curlet))))
</pre>


<blockquote>


<div class="indented">

<p>eval's environment argument is handy when implementing break and trace:
</p>
<pre>
(define *breaklet* #f)
(define *step-hook* (make-hook 'code 'e))

(define-macro* (trace/break code . break-points)
  (define (caller tree)
    (if (pair? tree)
	(cons 
	 (if (pair? (car tree))
	     (if (and (symbol? (caar tree))
		      (procedure? (symbol-&gt;value (caar tree))))
		 (if (member (car tree) break-points)
		     `(__break__ ,(caller (car tree)))
		     `(__call__ ,(caller (car tree))))
		 (caller (car tree)))
	     (car tree))
	 (caller (cdr tree)))
	tree))
  `(call-with-exit (lambda (__top__) ,(caller code))))

(define (go . args)
  (and (let? *breaklet*)
       (apply (*breaklet* 'go) args)))

(define (clear-break)
  (set! *breaklet* #f))

(define-macro (__call__ code)
  `(*step-hook* ',code (curlet)))

(define-macro (__break__ code)
  `(begin
     (call/cc
      (lambda (go)
	(set! *breaklet* (curlet))
	(__top__ (format #f "break at: ~A~%" ',code))))
     ,code))

(set! (hook-functions *step-hook*) 
      (list (lambda (hook)
	      (set! (hook 'result) (<em class=red>eval (hook 'code) (hook 'e)</em>)))))

(set! ((funclet *step-hook*) 'end)
      (list (lambda (hook)
	      (define (uncaller tree)
		(if (pair? tree)
		    (cons 
		     (if (and (pair? (car tree))
			      (memq (caar tree) '(__call__ __break__)))
			 (uncaller (cadar tree))
			 (uncaller (car tree)))
		     (uncaller (cdr tree)))
		    tree))
	      (format (current-output-port) ": ~A -&gt; ~A~40T~A~%" 
		      (uncaller (hook 'code)) 
		      (hook 'result)
		      (if (and (not (eq? (hook 'e) (rootlet)))
			       (not (defined? '__top__ (hook 'e))))
			  (map values (hook 'e)) 
			  "")))))

;;; (trace/break (let ((a (+ 3 1)) (b 2)) (if (&gt; (* 2 a) b) 2 3)))
;;; (trace/break (let ((a (+ 3 1)) (b 2)) (if (&gt; (* 2 a) b) 2 3)) (* 2 a))
</pre>
</div>

</blockquote>


<div class="header" id="IO"><h4>IO and other OS functions</h4></div>


<p>Besides files, ports can also represent strings and functions.  The string port functions
are:
</p>

<pre class="indented">
(with-output-to-string thunk)         ; open a string port as current-output-port, call thunk, return string
(with-input-from-string string thunk) ; open string as current-input-port, call thunk
(call-with-output-string proc)        ; open a string port, apply proc to it, return string
(call-with-input-string string proc)  ; open string as current-input-port, apply proc to it
(open-output-string)                  ; open a string output port
(get-output-string port clear)        ; return output accumulated in the string output port
(open-input-string string)            ; open a string input port reading string
</pre>

<pre class="indented">
&gt; (let ((result #f) 
        (p (<em class=red>open-output-string</em>)))
    (format p "this ~A ~C test ~D" "is" #\a 3)
    (set! result (<em class=red>get-output-string</em> p))
    (close-output-port p)
    result)
<em class="gray">"this is a test 3"</em>
</pre>

<p>In get-output-string, if the optional 'clear' argument is #t, the port is cleared (the default in r7rs I think).
Other functions:
</p>

<ul>
<li>read-byte and write-byte: binary IO.
<li>read-line: line-at-a-time reads, optional third argument #t to include the newline.
<li>current-error-port, set-current-error-port
</ul>

<p>The variable (*s7* 'print-length) sets
the upper limit on how many elements of a sequence are printed by object-&gt;string and format.
The end-of-file object is #&lt;eof&gt;. 
When running s7 behind a GUI, you often want input to come from and output to go to
arbitrary widgets. The function ports provide a way to redirect IO in C.  See <a href="#functionportexample">below</a>
for an example.
</p>


<blockquote>

<div class="indented">

<p>
The default IO ports are *stdin*, *stdout*, and *stderr*.
*stderr* is useful if you want to make sure output is flushed out immediately.
The default output port is *stdout* which buffers output until a newline is seen.
In most REPLs, the input port is set up by the REPL, so you need to use
*stdin* if you want to read from the terminal instead:  
</p>

<pre class="indented">
&gt; (read-char)
<em class="gray">#&lt;eof&gt;</em>
&gt; (read-char *stdin*)
a          ; here s7 waited for me to type "a" in the terminal
<em class="gray">#\a</em>        ; this is the REPL reporting what read-char returned
</pre>

</div>


<div class="indented">
<p>An environment can be treated as an IO port, providing what Guile calls a "soft port":
</p>

<pre class="indented">
(define (call-with-input-vector v proc)
  (let ((i -1))
    (proc (openlet (inlet 'read (lambda (p) (v (set! i (+ i 1)))))))))
</pre>

<p>Here the IO port is an open environment that redefines the "read" function so that it
returns the next element of a vector.  See stuff.scm for call-with-output-vector.
The "proc" argument above can also be a macro, giving you a kludgey way to get around
the dumb "lambda".  Here are more useful examples:
</p>
<pre class="indented">
(openlet          ; a soft port for format that sends its output to *stderr* and returns the string
  (inlet 'format (lambda (port str . args)
 	           (display (apply format #f str args) *stderr*))))

(define (open-output-log name)
  ;; return a soft output port that does not hold its output file open
  (define (logit name str)
    (let ((p (open-output-file name "a")))
      (display str p)
      (close-output-port p)))
  (openlet 
   (inlet :name name
	  :format (lambda (p str . args) (logit (p 'name) (apply format #f str args)))
	  :write (lambda (obj p)         (logit (p 'name) (object-&gt;string obj #t)))
	  :display (lambda (obj p)       (logit (p 'name) (object-&gt;string obj #f)))
	  :write-string (lambda (str p)  (logit (p 'name) str))
	  :write-char (lambda (ch p)     (logit (p 'name) (string ch)))
	  :newline (lambda (p)           (logit (p 'name) (string #\newline)))
	  :close-output-port (lambda (p) #f)
	  :flush-output-port (lambda (p) #f))))
</pre>
</div>


<div class="indented">

<p>binary-io.scm in the Snd package has functions that read and write integers and floats in
both endian choices in a variety of sizes.
</p>

</div>
</blockquote>

<p>If the compile time switch WITH_SYSTEM_EXTRAS is 1, several additional OS-related and
file-related functions are built-in.  This is work in progress; currently this switch
adds:
</p>

<pre class="indented">
(directory? str)         ; return #t if str is the name of a directory
(file-exists? str)       ; return #t if str names an existing file
(delete-file str)        ; try to delete the file, return 0 is successful, else -1
(getenv var)             ; return the value of an environment variable: (getenv "HOME")
(directory-&gt;list dir)    ; return contents of directory as a list of strings (if HAVE_DIRENT_H)
(system command)         ; execute command
</pre>

<p>But maybe this is not needed; see <a href="#cload">cload.scm</a> below for
a more direct approach.
</p>


<div class="header" id="errors"><h4>error handling</h4></div>

<pre class="indented">
(error tag . info)           ; signal an error of type tag with addition information
(catch tag body err)         ; if error of type tag signalled in body (a thunk), call err with tag and info
(throw tag . info)           ; jump to corresponding catch
</pre>

<p>s7's error handling mimics that of Guile.  An error is signalled
via the error function, and can be trapped and dealt with via <em class=def id="catch">catch</em>.
</p>

<pre class="indented">
&gt; (<em class=red>catch</em> 'wrong-number-of-args
    (lambda ()     ; code protected by the catch
      (abs 1 2))
    (lambda args   ; the error handler
      (apply format #t (cadr args))))
<em class="gray">"abs: too many arguments: (1 2)"</em>
&gt; (<em class=red>catch</em> 'division-by-zero
    (lambda () (/ 1.0 0.0))
    (lambda args (string-&gt;number "inf.0")))
<em class="gray">inf.0</em>

(define-macro (catch-all . body)
  `(<em class=red>catch</em> #t (lambda () ,@body) (lambda args args)))
</pre>

<p>
catch has 3 arguments: a tag indicating what error to catch (#t = anything),
the code, a thunk, that the catch is protecting, and the function to call
if a matching error occurs during the evaluation of the thunk.  The error handler
takes a rest argument which will hold whatever the error function chooses to pass it.
The error function itself takes at least 2 arguments, the error type, a symbol,
and the error message.  There may also be other arguments describing the error.
The default action, in the absence of any catch, is to treat the message as
a format control string, apply format to it and the other arguments, and
send that info to the current-error-port.
</p>


<blockquote>
<div class="indented">
<p>Normally when reading a file, we have to check for #&lt;eof&gt;, but we can let s7
do that:
</p>

<pre>
(define (copy-file infile outfile)
  (call-with-input-file infile
    (lambda (in)
      (call-with-output-file outfile
	(lambda (out)
	  (<em class=red>catch</em> 'wrong-type-arg   ; s7 raises this error if write-char gets #&lt;eof&gt;
	    (lambda () 
	      (do () ()            ; read/write until #&lt;eof&gt;
		(write-char (read-char in) out)))
	    (lambda err 
	      outfile)))))))
</pre>

<p>catch is not limited to error handling:
</p>

<pre class="indented">
(define (map-with-exit func . args)
  ;; map, but if early exit taken, return the accumulated partial result
  ;;   func takes escape thunk, then args
  (let* ((result ())
	 (escape-tag (gensym))
	 (escape (lambda () (throw escape-tag))))
    (<em class=red>catch</em> escape-tag
      (lambda ()
	(let ((len (apply max (map length args))))
	  (do ((ctr 0 (+ ctr 1)))
	      ((= ctr len) (reverse result))      ; return the full result if no throw
	    (let ((val (apply func escape (map (lambda (x) (x ctr)) args))))
	      (set! result (cons val result))))))
      (lambda args
	(reverse result))))) ; if we catch escape-tag, return the partial result

(define (truncate-if func lst)
  (map-with-exit (lambda (escape x) (if (func x) (escape) x)) lst))

&gt; (truncate-if even? #(1 3 5 -1 4 6 7 8))
<em class="gray">(1 3 5 -1)</em>
</pre>

<p>But this is less useful than map (it can't map over a hash-table for example),
and is mostly reimplementing built-in code.  Perhaps s7 should have an extension
of map (and more usefully, for-each) that is patterned after dynamic-wind:
<code>(dynamic-for-each init-func main-func end-func . args)</code> where init-func
is called with one argument, the length of the shortest sequence argument (for-each
and map know this in advance); main-func takes n arguments where n matches
the number of sequences passed; and end-func is called even if a jump out of main-func
occurs (like dynamic-wind in this regard).  In the dynamic-map case, the end-func
takes one argument, the current, possibly partial, result list.  dynamic-for-each
then could easily (but maybe not efficiently) implement generic functions such as -&gt;list, -&gt;vector, and
-&gt;string (converting any sequence into a sequence of some other type).
map-with-exit would be
</p>
<pre class="indented">
(define (map-with-exit func . args) 
  (let ((result ()))
    (call-with-exit
      (lambda (quit)
        (apply dynamic-map #f ; no init-func in this case
               (lambda main-args 
                 (apply func quit main-args)) 
               (lambda (res) 
                 (set! result res))
               args)))
    result))
</pre>
</div>

<div class="indented">
<p>With all the lambda boilerplate, nested catches are hard to read:
</p>
<pre class="indented">
(catch #t
  (lambda ()
    (catch 'division-by-zero
      (lambda ()
	(catch 'wrong-type-arg
	  (lambda () 
	    (abs -1))
	  (lambda args (format () "got a bad arg~%") -1)))
      (lambda args 0)))
  (lambda args 123))
</pre>

<p>Perhaps we need a macro:
</p>

<pre class="indented">
(define-macro (catch-case clauses . body)
  (let ((base `(lambda () ,@body)))
    (for-each (lambda (clause)
	        (let ((tag (car clause)))
	          (set! base `(lambda () 
			        (catch ',(or (eq? tag 'else) tag)
			          ,base 
			          ,@(cdr clause))))))
	      clauses)
    (caddr base)))

;;; the code above becomes:
(catch-case ((wrong-type-arg   (lambda args (format () "got a bad arg~%") -1))
	     (division-by-zero (lambda args 0))
	     (else             (lambda args 123)))
  (abs -1))
</pre>

<p>This is similar to r7rs scheme's "guard", but I don't want a pointless thunk for the body of the catch.
Along the same lines:
</p>
<pre class="indented">
(define (catch-if test func err)
  (catch #t
    func
    (lambda args
      (apply (if (test (car args)) err throw) args)))) ; if not caught, re-raise the error via throw

(define (catch-member lst func err)
  (catch-if (lambda (tag) (member tag lst)) func err))

(define-macro (catch* clauses . error) 
  ;; try each clause until one evaluates without error, else error:
  ;;    (macroexpand (catch* ((+ 1 2) (- 3 4)) 'error))
  ;;    (catch #t (lambda () (+ 1 2)) (lambda args (catch #t (lambda () (- 3 4)) (lambda args 'error))))
  (define (builder lst)
    (if (null? lst)
	(apply values error)
	`(catch #t (lambda () ,(car lst)) (lambda args ,(builder (cdr lst))))))
  (builder clauses))
</pre>
</div>

<!--
(define (or-catch . funks)
  (call-with-exit
   (lambda (return)
     (for-each
      (lambda (f)
	(catch #t
	  (lambda ()
	    (return (f)))
	  (lambda args
	    (case (car args)
	      ((wrong-type-arg) ...)
	      (...)
	      (else (apply throw args))))))
      funks))))
-->       

</blockquote>

<p>When an error is encountered, and when s7 is interrupted via <a href="#beginhook">begin_hook</a>, 
(<em class=def id="owlet">owlet</em>) returns an environment that contains
additional info about that error:
</p>

<ul>
<li>error-type: the error type or tag, e.g. 'division-by-zero
<li>error-data: the message or information passed by the error function
<li>error-code: the code that s7 thinks triggered the error
<li>error-line: the line number of that code
<li>error-file: the file name of that code
<li>error-history: previous evaluations leading to the error (a circular list)
</ul>

<p>The error-history field depends on the compiler flag WITH_HISTORY.  See ow! in
stuff.scm for one way to display this data.  The *s7* field 'history-size sets the size of the buffer.
</p>

<blockquote>


<div class="indented">

<p>To find a variable's value at the point of the error: <code>((owlet) var)</code>.
To list all the local bindings from the error outward:
</p>

<pre class="indented">
(do ((e (outlet (owlet)) (outlet e))) 
    ((eq? e (rootlet))) 
  (format () "~{~A ~}~%" e))
</pre>

<p>To see the current s7 stack, <code>(stacktrace)</code>.  You'll probably
want to use this in conjunction with *error-hook*.
To evaluate the error handler in the environment of the error:
</p>

<pre class="indented">
(let ((x 1))
  (catch #t
	 (lambda ()
	   (let ((y 2))
	     (error 'oops)))
	 (lambda args
	   (with-let (sublet (owlet) :args args)    ; add the error handler args
	     (list args x y)))))    ; we have access to 'y'
</pre>

<p>To limit the maximum size of the stack, set (*s7* 'max-stack-size).
</p>
</div>


</blockquote>


<p>The hook <em class=def id="errorhook">*error-hook*</em> provides a way to specialize error reporting.
Its arguments are named 'type and 'data.
</p>

<pre class="indented">
(set! (hook-functions *error-hook*) 
      (list (lambda (hook) 
              (apply format *stderr* (hook 'data)) 
              (newline *stderr*))))
</pre>

<p><em class=def id="readerrorhook">*read-error-hook*</em> provides two hooks into the reader.
A major problem when reading code written for other Schemes is that each Scheme provides
a plethora of idiosyncratic #-names (even special character names), and \ escapes in string
constants.  *read-error-hook* provides a way to handle these weird cases.  If a #-name
is encountered that s7 can't deal with, *read-error-hook* is called with two arguments,
 #t and the string representing the constant.  If you set (hook 'result), that result is
returned to the reader.  Otherwise a 'read-error is raised and you drop into the error handler.
Similarly, if some bizaare \ use occurs, *read-error-hook* is called with two arguments,
#f and the offending character.  If you return a character, it is passed to the reader;
otherwise you get an error.  lint.scm has an example.
</p>

<p>
There is a break macro defined in Snd (snd-xen.c)
which allows you to stop at some point, then evaluate arbitrary expressions in that context.
</p>


<div class="indented">
<p>The s7-built-in catch tags are 'wrong-type-arg, 'syntax-error, 'read-error, 
'out-of-memory, 'wrong-number-of-args, 'format-error, 'out-of-range, 'division-by-zero, 'io-error, and 'bignum-error.  
</p>
</div>


<div class="header" id="autoload"><h4>autoload</h4></div>

<!-- INDEX autoload:autoload -->
<p>
If s7 encounters an unbound variable, it first looks to see if it has any autoload information about it.
This info can be declared via <em class=def>autoload</em>, a function of two arguments, the
symbol that triggers the autoload, and either a filename or a function.  If a filename, s7
loads that file; if a function, it is called with one argument, the current (calling) environment.
</p>

<pre class="indented">
(autoload 'channel-distance "dsp.scm") 
;; now if we subsequently call channel-distance but forget to load "dsp.scm" first,
;;   s7 loads "dsp.scm" itself, and uses its definition of channel-distance.
;;   The C-side equivalent is s7_autoload.

;; here is the cload.scm case, loading j0 from the math library if it is called:
(autoload 'j0
	  (lambda (e)
	    (unless (provided? 'cload.scm)
	      (load "cload.scm"))
	    (c-define '(double j0 (double)) "" "math.h")
	    (varlet e 'j0 j0)))
</pre>

<p>The entity (hash-table or environment probably) that holds the autoload info is named *autoload*.
If after checking autoload, the symbol is still unbound, s7 calls
<em class=def id="unboundvariablehook">*unbound-variable-hook*</em>.
The offending symbol is named 'variable in the hook environment.
If after running *unbound-variable-hook*, the symbol is still unbound,
s7 calls the error handler.
</p>

<p>The autoloader knows about s7 environments used as libraries, so, for example,
you can <code>(autoload 'j0 "libm.scm")</code>, then use j0 in scheme code. The first
time s7 encounters j0, j0 is undefined, so
s7 loads libm.scm.  That load returns the C math library as the environment *libm*.
s7 then automatically looks for j0 in *libm*, and defines it for you.
So the result is the same as if you had defined j0 yourself in C code.
You can use the r7rs library mechanism here, or with-let, or
whatever you want! (In Snd, libc, libm, libdl, and libgdbm are automatically
tied into s7 via autoload, so if you call, for example, frexp, libm.scm
is loaded, and frexp is exported from the *libm* environment, then the
evaluator soldiers on, as if frexp had always been defined in s7).
You can also import all of (say) gsl into the current environment
via <code>(varlet (curlet) *libgsl*)</code>.
</p>


<div class="header" id="constants"><h4>define-constant, constant?, symbol-access</h4></div>

<p><em class=def id="defineconstant">define-constant</em> defines a constant and 
<em class=def id="constantp">constant?</em> returns #t if its argument
is a constant.  A constant in s7 is really constant: it can't be set or rebound.
</p>

<pre class="indented">
&gt; (define-constant var 32)
<em class="gray">var</em>
&gt; (set! var 1)
<em class="gray">;set!: can't alter immutable object: var</em>
&gt; (let ((var 1)) var)
<em class="gray">;can't bind or set an immutable object: var, line 1</em>
</pre>

<p>This has the possibly surprising side effect that previous uses of the constant name
become constants:
</p>

<pre class="indented">
(define (func a) (let ((cvar (+ a 1))) cvar))
(define-constant cvar 23)
(func 1)
;can't bind or set an immutable object: cvar
</pre>

<p>So, obviously, choose unique names for your constants, or don't use define-constant.
A function can also be a constant.  In some cases, the optimizer can take advantage
of this information to speed up function calls.
</p>

<p>Constants are very similar to things such as keywords (no set, always return itself as its value),
variable trace (informative function upon set or keeping a history of past values), typed variables (restricting a
variable's values or doing automatic conversions upon set), and notification upon set (either in Scheme
or in C; I wanted this many years ago in Snd).  The notification function is especially useful if
you have a Scheme variable and want to reflect any change in its value immediately in C (see <a href="#notify">below</a>).
In s7, <em class=def id="symbolaccess">symbol-access</em> sets this function.
</p>

<p>Each environment is a set of symbols and their associated values.  symbol-access places a function (or macro) between a symbol
and its value in a given environment.  The accessor function takes two arguments, the symbol and the new value, and
returns the value that is actually set.  For example, the function can ensure that a variable is always an integer:
</p>

<pre class="indented">
(define e      ; save environment for use below
  (let ((x 3)  ; will always be an integer
	(y 3)  ; will always keep its initial value
	(z 3)) ; will report set!

    (set! (symbol-access 'x) (lambda (s v) (if (integer? v) v x)))
    (set! (symbol-access 'y) (lambda (s v) y))
    (set! (symbol-access 'z) (lambda (s v) (format *stderr* "z ~A -&gt; ~A~%" z v) v))
  
    (set! x 3.3) ; x does not change because 3.3 is not an integer
    (set! y 3.3) ; y does not change
    (set! z 3.3) ; prints "z 3 -&gt; 3.3" 
    (curlet)))

&gt; e
<em class="gray">(inlet 'x 3 'y 3 'z 3.3)</em>
&gt;(begin (set! (e 'x) 123) (set! (e 'y) #()) (set! (e 'z) #f))
;; prints "z 3.3 -&gt; #f"
&gt; e
<em class="gray">(inlet 'x 123 'y 3 'z #f)</em>
&gt; (define-macro (reflective-let vars . body)
    `(let ,vars
       ,@(map (lambda (vr)
	        `(set! (symbol-access ',(car vr))
		       (lambda (s v)
		         (format *stderr* "~S -&gt; ~S~%" s v)
		         v)))
	      vars)
       ,@body))
<em class="gray">reflective-let</em>
&gt; (reflective-let ((a 1)) (set! a 2))
<em class="gray">2</em>     ; prints "a -&gt; 2"
&gt;(let ((a 0))
     (set! (symbol-access 'a)
      (let ((history (make-vector 3 0))
	    (position 0))
	(lambda (s v)
	  (set! (history position) v)
	  (set! position (+ position 1))
	  (if (= position 3) (set! position 0))
	  v)))
     (set! a 1)
     (set! a 2)
     ((funclet (symbol-access 'a)) 'history))
<em class="gray">#(1 2 0)</em>
</pre>

<p>Or implement reactive programming:
</p>
<pre class="indented">
(let ((a 1)
      (b 2)
      (c 3))
  (set! (symbol-access 'b) (lambda (s v) (set! a (+ v c)) v))
  (set! (symbol-access 'c) (lambda (s v) (set! a (+ b v)) v))
  (set! a (+ b c)) ; a will be updated if b or c is set
  (set! b 4)
  (set! c 5)
  a)               ; a is 9 = (+ 4 5)
</pre>


<details>
<summary class="indented">reactive-let</summary>
<div class="indented">

<p>stuff.scm has reactive-set!, reactive-vector, reactive-let, reactive-let*, and reactive-lambda*:
</p>
<pre class="indented">
&gt; (let ((-a- 1)
        (b 2))
    (<em class=red>reactive-set!</em> -a- (* b 2))
    (set! b 3)
    -a-)
<em class="gray">6</em>
&gt; (let ((a 1)) 
    (let ((v (<em class=red>reactive-vector</em> a (+ a 1) 2))) 
      (set! a 4) 
      v))
<em class="gray">#(4 5 2)</em>
&gt; (let ((a 1)) 
    (<em class=red>reactive-let</em> ((-b- (+ a 1))) ; if 'a changes, '-b- does too
    (set! a 3)                  ;   so '-b- is now 4
    -b-))
<em class="gray">4</em>
&gt; (let ((a 1))
    (<em class=red>reactive-lambda*</em> (s v)
      (format *stderr* "~S -&gt; ~S~%" s v))
    (set! a 3))
<em class="gray">"a -&gt; 3"</em>
</pre>

<p>In the reactive-let example, the macro notices that '-b- depends on 'a, so it sets up a symbol-access on 'a
so that <code>(set! a 3)</code> triggers <code>(set! -b- (+ a 1))</code>.  I'm using -name- to distinguish
the variables that can change value at any time; in the Lisp community, +name+ is often used to mark a constant,
so this seems like a natural convention.
</p>

<p>Here's the standard example of following the mouse (assuming you're using Snd and glistener):
</p>
<pre class="indented">
(let ((*mouse-x* 0) (*mouse-y* 0)
      (x 0) (y 0))

  (reactive-set! x (let ((val (round *mouse-x*))) 
		     (format *stderr* "mouse: ~A ~A~%" x y) 
		     val))
  (reactive-set! y (round *mouse-y*))

  (g_signal_connect (G_OBJECT (listener-text-widget *listener*)) "motion_notify_event" 
		    (lambda (w e d) 
		      (let ((mxy (cdr (gdk_event_get_coords (GDK_EVENT e)))))
			(set! *mouse-x* (car mxy))
			(set! *mouse-y* (cadr mxy))))))
</pre>

<!--
(let ((*mouse-x* 0) (*mouse-y* 0))

  (g_signal_connect (G_OBJECT (listener-text-widget *listener*)) "motion_notify_event" 
		    (lambda (w e d) 
		      (let ((mxy (gdk_event_get_coords (GDK_EVENT e))))
			(set! *mouse-x* (cadr mxy))
			(set! *mouse-y* (caddr mxy)))))

  (with-accessors (*mouse-x* *mouse-y*)
    (let ((x 0) (y 0))
      (reactive-set! x (let ((val (round *mouse-x*))) 
		       (format *stderr* "mouse: ~A ~A~%" x y) 
		       val))
      (reactive-set! y (round *mouse-y*))
      (gtk_main))))
-->

<p>reactive-lambda* is aimed at library consistency.  Say we have the following library
that wants A to always be half B:
</p>

<pre class="indented">
(define (make-library)
  (let ((A 1.0)
	(B 2.0))
    (reactive-lambda* (s v)
      (case s
	((A) (set! B (* 2 v)))
	((B) (set! A (/ v 2)))))
    (define (f1 x) 
      (+ A (* B x)))
    (curlet)))

(with-let (make-library)
  (format *stderr* "(f1 3): ~A~%" (f1 3))
  (set! A 3.0)
  (format *stderr* "A: ~A, B: ~A, (f1 3): ~A~%" A B (f1 3))
  (set! B 4.0)
  (format *stderr* "A: ~A, B: ~A, (f1 3): ~A~%" A B (f1 3)))
</pre>

<p>reactive-lambda* sets up accessors on the library's top-level variables
that call the lambda body if one of the variables is set.
</p>

<p>None of these macros does the right thing yet; I'm sort of following my nose.
</p>

</div>
</details>


<div class="header" id="miscellanea"><h4>marvels and curiousities</h4></div>

<p>
<b><em class=def id="loadpath">*load-path*</em></b> is a list of directories to search when loading a file.
<b><em class=def id="loadhook">*load-hook*</em></b> is a hook whose functions are called just before a file is loaded.  
The hook function argument, named 'name, is the filename.
While loading, the <em class=def id="portfilename">port-filename</em> and 
<em class=def id="portlinenumber">port-line-number</em> of the current-input-port can tell you
where you are in the file.  This data is available after loading via <em class=def id="pairlinenumber">pair-line-number</em>
and <em class=def id="pairfilename">pair-filename</em>.  port-line-number is settable (for fancy *#readers*).
</p>

<pre class="indented">
(set! (hook-functions *load-hook*)
       (list (lambda (hook) 
               (format () "loading ~S...~%" (hook 'name)))))

(set! (hook-functions *load-hook*) 
      (cons (lambda (hook) 
              (format *stderr* "~A~%" 
                (system (string-append "./snd lint.scm -e '(begin (lint \"" (hook 'name) "\") (exit))'") #t)))
            (hook-functions *load-hook*)))
</pre>

<p>Here's a *load-hook* function that adds the loaded file's directory
to the *load-path* variable so that subsequent loads don't need to specify
the directory:
</p>

<pre class="indented">
(set! (hook-functions <em class=red>*load-hook*</em>)
  (list (lambda (hook)
          (let ((pos -1)
                (filename (hook 'name)))
            (do ((len (length filename))
                 (i 0 (+ i 1)))
	        ((= i len))
	      (if (char=? (filename i) #\/)
	          (set! pos i)))
            (if (positive? pos)
	        (let ((directory-name (substring filename 0 pos)))
	          (if (not (member directory-name <em class=red>*load-path*</em>))
		      (set! <em class=red>*load-path*</em> (cons directory-name *load-path*)))))))))
</pre>


<div class="separator"></div>

<p>As in Common Lisp, <b><em class=def id="featureslist">*features*</em></b> is a list describing what is currently loaded into s7.  You can
check it with the <b>provided?</b> function, or add something to it with <b>provide</b>.  In my version of Snd,
at startup *features* is:
</p>

<pre class="indented">
&gt; *features*
<em class="gray">(snd-16.7 snd15 snd audio snd-s7 snd-gtk gsl alsa gtk2 xg clm6 clm sndlib linux
 dlopen complex-numbers system-extras ratio s7-3.26 s7) </em>
&gt; (provided? 'gsl)
<em class="gray">#t</em>
</pre>

<p>The other side of <code>provide</code> is <em class=def id="requires7">require</em>.
<code>(require . things)</code> finds each thing
(via <a href="#autoload">autoload</a>), and if that thing has not already been loaded,
loads the associated file.  <code>(require integrate-envelope)</code>
loads "env.scm", for example; in this case it is equivalent to
simply using integrate-envelope, but if placed at the start of
a file, it documents that you're using that function.
In the more normal use, <code>(require snd-ws.scm)</code>
looks for the file that has <code>(provide 'snd-ws.scm)</code>
and if it hasn't already been loaded, loads it ("ws.scm" in this case).
To add your own files to this mechanism, add the provided symbol via <a href="#autoload">autoload</a>.
Since load can take an environment argument, *features* and its friends follow block structure.
So, for example, (let () (require stuff.scm) ...) loads "stuff.scm" into the local environment,
not globally.
</p>

<div class="indented">
<p>*features* is an odd variable: it is spread out across the chain of environments, and
can hold features in an intermediate environment that aren't in subsequent (nested) values.
One simple way this can happen is to load a file in a let, but cause the load to happen
at the top level.  The provided entities get added to the top-level *features* value,
not the current let's value, but they are actually accessible locally.  So *features*
is a merge of all its currently accessible values, vaguely like call-next-method in
CLOS.  We can mimic this behavior:
</p>
<pre class="indented">
(let ((x '(a)))
  (let ((x '(b)))
    (define (transparent-memq sym var e)
      (let ((val (symbol-&gt;value var e)))
	(or (and (pair? val)
		 (memq sym val))
	    (and (not (eq? e (rootlet)))
		 (transparent-memq sym var (outlet e))))))
    (let ((ce (curlet)))
      (list (transparent-memq 'a 'x ce)
	    (transparent-memq 'b 'x ce)
	    (transparent-memq 'c 'x ce)))))

'((a) (b) #f)
</pre>    
</div>

<!--
(let ((spread-function (lambda (x e) (+ x 1))))
  (let ((spread-function (lambda (x e) (+ x 2))))
    (let ((x 3))
      (define (spread-function x e)
	(let ((val x))
	  (do ((e1 e (outlet e1)))
	      ((eq? e1 (rootlet)) val)
	    (let ((f (symbol->value 'spread-function e1)))
	      (if (procedure? f)
		  (set! val (f val (rootlet))))))))
      (spread-function x (curlet)))))
 6
-->


<div class="separator"></div>

<p>Multi-line and in-line comments can be enclosed in #| and |#.
<code>(+ #| add |# 1 2)</code>.
</p>

<div class="indented">
<p>Leaving aside this case and the booleans, #f and #t, you can specify your own handlers for 
tokens that start with "#".  <b><em class=def id="sharpreaders">*#readers*</em></b> is a list of pairs: <code>(char . func)</code>.
"char" refers to the first character after the sharp sign (#). "func" is a function of
one argument, the string that follows the #-sign up to the next delimiter.  "func" is called
when #&lt;char&gt; is encountered.  If it returns something other than #f, the #-expression
is replaced with that value.  Scheme has several predefined #-readers for cases such
as #b1, #\a, #i123, and so on, but you can override these if you like.  If the string
passed in is not the complete #-expression, the function can use read-char or read to get the
rest.  Say we'd like #t&lt;number&gt; to interpret the number in base 12:
</p>

<pre class="indented">
(set! *#readers* (cons (cons #\t (lambda (str) (string-&gt;number (substring str 1) 12))) *#readers*))

&gt; #tb
<em class="gray">11</em>
&gt; #t11.3
<em class="gray">13.25</em>
</pre>

<p>Or have #c(real imag) be read as a complex number:
</p>

<pre class="indented">
(set! *#readers* (cons (cons #\c (lambda (str) (apply complex (read)))) *#readers*))

&gt; #c(1 2)
<em class="gray">1+2i</em>
</pre>

<p>Here's a reader macro for read-time evaluation:
</p>

<pre class="indented">
(set! *#readers*
  (cons (cons #\. (lambda (str)
		    (and (string=? str ".") (eval (read)))))
	*#readers*))

&gt; '(1 2 #.(* 3 4) 5)
<em class="gray">(1 2 12 5)</em>
</pre>


<p>And a reader that implements #[...]# for literal hash-tables:
</p>

<pre class="indented">
&gt; (set! *#readers* 
    (list (cons #\[ (lambda (str)
		      (let ((h (make-hash-table)))
		        (do ((c (read) (read)))
		            ((eq? c ']#) h) ; ]# is a symbol from the reader's point of view
		          (set! (h (car c)) (cdr c))))))))
<em class="gray">((#\[ . #&lt;lambda (str)&gt;))</em>
&gt; #[(a . 1) (b . #[(c . 3)]#)]#
<em class="gray">(hash-table '(b . (hash-table '(c . 3))) '(a . 1))</em>
</pre>


<p>To return no value from a reader, use <code>(values)</code>.
</p>
<pre class="indented">
&gt; (set! *#readers* (cons (cons #\; (lambda (str) (if (string=? str ";") (read)) (values))) *#readers*))
<em class="gray">((#\; . #&lt;lambda (str)&gt;))</em>
&gt; (+ 1 #;(* 2 3) 4)
<em class="gray">5</em>
</pre>
<p>Here is CL's #+ reader:
</p>
<pre class="indented">
(define (sharp-plus str)
  ;; str here is "+", we assume either a symbol or an expression involving symbols follows
  (let ((e (if (string=? str "+")
		(read)                                ; must be #+(...)
		(string-&gt;symbol (substring str 1))))  ; #+feature
	(expr (read)))  ; this is the expression following #+
    (if (symbol? e)
        (if (provided? e)
	    expr
	    (values))
	(if (not (pair? e))
	    (error "strange #+ chooser: ~S~%" e)
	    (begin      ; evaluate the #+(...) expression as in cond-expand
	      (define (traverse tree)
		(if (pair? tree)                                             
		    (cons (traverse (car tree))                             
			  (if (null? (cdr tree)) () (traverse (cdr tree))))
		    (if (memq tree '(and or not)) tree                 
			(and (symbol? tree) (provided? tree)))))
	      (if (eval (traverse e))
		  expr
		  (values)))))))
</pre>
<p>See also the <a href="#circularlistreader">#n=</a> reader below.</p>
</div>

<div class="separator"></div>

<p id="makelist">(<b>make-list</b> length (initial-element #f)) returns a list of 'length' elements defaulting to 'initial-element'.
</p>

<div class="separator"></div>

<pre class="indented">
(<em class=def id="charposition">char-position</em> char-or-string searched-string (start 0))
(<em class=def id="stringposition">string-position</em> substring searched-string (start 0))
</pre>

<p>
<b>char-position</b> and <b>string-position</b> search a string for the occurrence of a character,
any of a set of characters, or a string.  They return either #f if none is found, or the position
within the searched string of the first occurrence.  The optional third argument sets where the
search starts in the second argument.
</p>

<p>If char-position's first argument is a string, it is treated as a set of characters, and
char-position looks for the first occurrence of any member of that set.
Currently, the strings involved are assumed to be C strings (don't expect embedded nulls
to work right in this context).
</p>

<pre class="indented">
(call-with-input-file "s7.c" ; report any lines with "static " but no following open paren
  (lambda (file)
    (let loop ((line (read-line file #t)))
      (or (eof-object? line)
	  (let ((pos (<em class=red>string-position</em> "static " line)))
	    (if (and pos
		     (not (<em class=red>char-position</em> #\( (substring line pos))))
 	        (if (&gt; (length line) 80)
		    (begin (display (substring line 0 80)) (newline))
		    (display line))))
	    (loop (read-line file #t)))))))
</pre>


<div class="separator"></div>

<p id="keywords">
Keywords exist mainly for define*'s benefit.  The keyword functions are:
<b>keyword?</b>, <b>make-keyword</b>, <b>symbol-&gt;keyword</b>, and <b>keyword-&gt;symbol</b>.
A keyword is a symbol that starts or ends with a colon. The colon
is considered to be a part of the symbol name.  A keyword is a constant that evaluates to itself.
</p>


<div class="separator"></div>

<pre class="indented">
(<em class=def id="symboltable">symbol-table</em>)
(<em class=def id="symboltovalue">symbol-&gt;value</em> sym (env (curlet)))
(<em class=def id="symboltodynamicvalue">symbol-&gt;dynamic-value</em> sym)
(<em class=def id="definedp">defined?</em> sym (env (curlet)) ignore-global-env)
</pre>

<p>
<code>defined?</code> returns #t if the symbol is defined in the environment:
</p>

<pre class="indented">
(define-macro (defvar name value) 
  `(unless (defined? ',name)
     (define ,name ,value)))
</pre>

<p>If ignore-global-env is #t, the search is confined to the given environment.
<code>symbol-&gt;value</code> returns the value (lexically) bound to the symbol, whereas <code>symbol-&gt;dynamic-value</code>
returns the value dynamically bound to it.  A variable can access both of its values:
</p>
<pre class="indented">
&gt; (let ((x 32))
  (define (gx) ; return both bindings of 'x
    (list x (<em class=red>symbol-&gt;value</em> 'x) (<em class=red>symbol-&gt;dynamic-value</em> 'x)))
  (let ((x 100))
    (let ((x 12))
      (values (gx))))) ; this 'values' looks dumb, it is dumb, but see my unconvincing explanantion <a href="#weaselwords">below</a>.
                       ;   briefly: "dynamic binding" in s7 is not "lexically apparent dynamic binding"
<em class="gray">(32 32 12)</em>
</pre>

<p>
<code>symbol-table</code> returns a vector containing the symbols currently in the symbol-table.
Here we scan the symbol table looking for any function that doesn't have documentation:
</p>

<pre class="indented">
(for-each 
   (lambda (sym)
     (if (<em class=red>defined?</em> sym)
         (let ((val (<em class=red>symbol-&gt;value</em> sym)))
           (if (and (procedure? val)
                    (string=? "" (procedure-documentation val)))
               (format *stderr* "~S " sym)))))
  (<em class=red>symbol-table</em>))
</pre>


<div class="indented">

<p>Here's a better example, an automatic software tester.
</p>

<pre class="indented">
(for-each 
  (lambda (sym)
    (if (<em class=red>defined?</em> sym)
	(let ((val (<em class=red>symbol-&gt;value</em> sym)))
          (if (procedure? val)
	      (let ((max-args (cdr (arity val))))
	        (if (or (&gt; max-args 4)
	   	        (memq sym '(exit abort)))
		    (format () ";skip ~S for now~%" sym)
		    (begin
		      (format () ";whack on ~S...~%" sym)
                      (let ((constants (list #f #t pi () 1 1.5 3/2 1.5+i)))
                        (let autotest ((args ()) (args-left max-args))
                          (catch #t (lambda () (apply func args)) (lambda any #f))
                          (if (&gt; args-left 0)
 	                      (for-each
	                        (lambda (c)
	                          (autotest (cons c args) (- args-left 1)))
	                        constants)))))))))))
  (<em class=red>symbol-table</em>))
</pre>
</div>

<details id="weaselwords">
<summary class="indented">more on dynamic values</summary>
<div class="indented">
<p>Why the useless 'values' in our dynamic binding example?  The short answer: tail calls.
The long winded one goes something like this.  symbol-&gt;dynamic-value searches the stack to find
the latest binding of its argument.  But because we want to support tail calls, "let" does not
push anything on the stack.  If we call a function as the last thing that happens in that let's body,
and it tries (internally) to access a dynamic binding, the let that launched the function no longer exists; it might already
be garbage collected, and it certainly isn't on the stack. Take our earlier example without the
'values':
</p>

<pre class="indented">
(let ((x 32))
  (define (gx) 
    (symbol-&gt;dynamic-value 'x))
  (let ((x 100))
    (gx)))
</pre>

<p>
This returns 32 because the <code>(x 100)</code> binding no longer exists anywhere when the gx body is evaluated.
So, in s7, the "dynamic binding" of x is the last binding of x that is accessible
to s7.  This may not be the last binding that we can see in the code text, but I don't see that as
an inconsistency. It's not lexical after all. 
Leaving aside this special case, so to speak,
dynamic binding does what you'd expect, even in the context of call/cc.  See s7test.scm for
the MIT-Scheme test of that interaction.
</p>

<p>There is another way to get the call-time value:
</p>
<pre class="indented">
(define-macro (elambda args . body)
  `(define-macro (,(gensym) ,@args)
    `((lambda* ,(append ',args `((*env* (curlet)))) 
        ,'(begin ,@body)) 
      ,,@args)))

(define efunc (elambda (x) (+ (*env* 'y) x))) ; add x to the run-time value of y
(let ((y 3)) (efunc 1)) ; returns 4
</pre>

<p>elambda does not suffer from the symbol-&gt;dynamic-value defects mentioned above, but
it's probably slower.  We have to wrap the function in a macro because lambda*'s argument default
values are evaluated in the definition environment, but we want the run-time environment.
</p>

<pre class="indented">
(define-macro* (rlambda args . body) ; lambda* but eval arg defaults in run-time env
  (let ((arg-names (map (lambda (arg) (if (pair? arg) (car arg) arg)) args))
	(arg-defaults (map (lambda (arg) (if (pair? arg) `(,(car arg) (eval ,(cadr arg))) arg)) args)))
    `(define-bacro* (,(gensym) ,@arg-defaults)
      `((lambda ,',arg-names ,'(begin ,@body)) ,,@arg-names))))
<!-- ( -->
(let ()
  (define rx (rlambda ((a x)) 
               (if (> a 0) 
                   (let ((x (- x 1))) ;:)
                     (rx))
                   0)))
  (let ((x 3))
    (rx)))
</pre>

<p>Now putting that idea with the procedure-annotation macro given earlier, and the __func__ business
(to get the caller's name), and taking care to handle default arguments correctly, we make a
macro that returns an anonymous macro that returns an anonymous function that &mdash; where was I?
</p>
<pre class="indented">
(define-macro (Display definition) ; (Display (define (f1 x) (+ x 1))) -&gt; an annotated version of f1
  (let ((func (caadr definition))
	(args (cdadr definition)))
    (let ((arg-names (map (lambda (a) (if (symbol? a) a (car a))) args))
	  (body (proc-walk `(begin ,@(cddr definition))))) ; from procedure-annotation above
      `(define ,func
	  (define-macro* (,(gensym) ,@args)
	    (let ((e (gensym))
		  (result (gensym)))
	      `((lambda* ,(append ',arg-names `((,e (curlet)))) ; the calling environment
		  (let ((,result '?))
		    (dynamic-wind
			(lambda ()    ; display the caller, if any, as well as the incoming arguments
			  (format *stderr* "(~A~{~^ ~A~})" ',',func 
				  (map (lambda (slot)
					 (if (gensym? (car slot)) (values) slot))
				       (outlet (outlet (curlet)))))
			  (let ((caller (eval '__func__ ,e)))
			    (if (not (eq? caller #&lt;undefined&gt;))
				(format *stderr* " ;called from ~S" caller))
			    (newline *stderr*)))
			(lambda ()   ; evaluate the original function's body with annotations
			  (set! ,result ,',body))
			(lambda ()   ; display the result (it is returned by the set! above)
			  (format *stderr* "    -&gt; ~S~%" ,result)))))
		,,@arg-names)))))))
</pre>
<p>stuff.scm has a more mature version of this macro.
</p>
</div>
</details>

<div class="separator"></div>

<p id="s7help"><b>help</b> tries to find information about its argument.
</p>

<pre class="indented">
&gt; (help 'caadar)
<em class="gray">"(caadar lst) returns (car (car (cdr (car lst)))): (caadar '((1 (2 3)))) -&gt; 2"</em>
</pre>


<div class="separator"></div>

<p id="s7gc"><b>gc</b> calls the garbage collector.  <code>(gc #f)</code> turns off the GC, and <code>(gc #t)</code> turns it on.
</p>


<div class="separator"></div>

<pre class="indented">
(<b><em class=def id="morallyequalp">morally-equal?</em></b> x y)
</pre>

<p>
Say we want to check that two different computations came to the same result, and that result might
involve circular structures.  Will equal? be our friend?
</p>

<pre class="indented">
&gt; (equal? 2 2.0)
<em class="gray">#f</em>
&gt; (let ((x +nan.0)) (equal? x x))
<em class="gray">#f</em>
&gt; (equal? .1 1/10)
<em class="gray">#f    </em>
&gt; (= .1 1/10)
<em class="gray">#f</em>
&gt; (= 0.0 0+1e-300i)
<em class="gray">#f</em>
</pre>

<p>No! We need an equality check that ignores epsilonic differences in real and
complex numbers, and knows that NaNs are equal for all practical purposes.
Leaving aside numbers, 
closed ports are not equal, yet nothing can be done with them.
#() is not equal to #2d().  And two closures are never equal, even if their
arguments, environments, and bodies are equal.
Since there might be circles, it is not easy to write
a replacement for equal? in Scheme.
So, in s7, if one thing is basically the same as
some other thing, they satisfy the function morally-equal?.
</p>

<pre class="indented">
&gt; (morally-equal? 2 2.0)        ; would "equal!?" be a better name?
<em class="gray">#t</em>
&gt; (morally-equal? 1/0 1/0)      ; NaN
<em class="gray">#t</em>
&gt; (morally-equal? .1 1/10)
<em class="gray">#t</em>                              ; floating-point epsilon here is 1.0e-15 or thereabouts
&gt; (morally-equal? 0.0 1e-300)
<em class="gray">#t</em>
&gt; (morally-equal? 0.0 1e-14)
<em class="gray">#f</em>                              ; its not always #t!
&gt; (morally-equal? (lambda () #f) (lambda () #f))
<em class="gray">#t</em>
</pre>

<p>The *s7* field morally-equal-float-epsilon sets the floating-point fudge factor.
I can't decide how bignums should interact with morally-equal?.  Currently,
if a bignum is involved, either here or in a hash-table, s7 uses equal?.
Finally, if either argument is an environment with a 'morally-equal? method,
that method is invoked.
</p>


<div class="separator"></div>

<p>
<b><em class=def id="expansion">define-expansion</em></b> defines a macro that expands at read-time.
It has the same syntax as
define-macro, and (in normal use) the same result, but it is much faster because it expands only once.
(See define-with-macros in s7test.scm for a way to expand macros in a function body at definition time).
Since the reader knows almost nothing
about the code it is reading, 
you need to make sure the expansion is defined at the top level and that its name is unique!
The reader does know about global variables, so:
</p>

<pre class="indented">
(define *debugging* #t)

(define-expansion (assert assertion)
  (if *debugging*          ; or maybe better, (eq? (symbol-&gt;value '*debugging*) #t)
      `(unless ,assertion
	 (format *stderr* "~A: ~A failed~%" __func__ ',assertion))
      (values)))
</pre>

<p>Now the assertion code is only present in the function body (or wherever)
if *debugging* is #t; otherwise assert expands into nothing.  Another very handy
use is to embed a source file line number into a message; see for example lint-format
in lint.scm.
Leaving aside
read-time expansion and splicing, the real difference between define-macro and define-expansion
is that the expansion's result is not evaluated.
I'm no historian, but I believe that means that define-expansion creates
a (gasp!) f*xpr.  In fact:
</p>

<pre>
(define-macro (define-f*xpr name-and-args . body)
  `(define ,(car name-and-args)
     (apply define-expansion 
       (append (list (append (list (gensym)) ',(cdr name-and-args))) ',body))))

&gt; (define-f*xpr (mac a) `(+ ,a 1))
<em class="gray">mac</em>
&gt; (mac (* 2 3))
<em class="gray">(+ (* 2 3) 1)</em>
</pre>

<p>
You can do something similar with a normal macro, or make the indirection explicit:
</p>

<pre class="indented">
&gt; (define-macro (fx x) `'(+ 1 ,x)) ; quote result to avoid evaluation
<em class="gray">fx</em>
&gt; (let ((a 3)) (fx a))
<em class="gray">(+ 1 a)</em>
&gt; (define-expansion (ex x) `(+ 1 ,x))
<em class="gray">ex</em>
&gt; (let ((x ex) (a 3)) (x a))       ; avoid read-time splicing
<em class="gray">(+ 1 a)</em>
&gt; (let ((a 3)) (ex a))             ; spliced in at read-time
<em class="gray">4</em>
</pre>

<p>As this example shows, the reader knows nothing about the program context,
so if it does not see a list whose first element is a expansion name, it does
not do anything special.  In the <code>(x a)</code> case above, the
expansion happens when the code is evaluated, and the expansion result
is simply returned, unevaluated.
</p>

<p>You can also use macroexpand to cancel the evaluation of a macro's expansion:
</p>
<pre>
(define-macro (rmac . args)
  (if (null? args)
      ()
      (if (null? (cdr args))
	  `(display ',(car args))
	  (list 'begin
                `(display ',(car args))
		(apply macroexpand (list (cons 'rmac (cdr args))))))))

&gt; (macroexpand (rmac a b c))
<em class="gray">(begin (display 'a) (begin (display 'b) (display 'c)))</em>
&gt; (begin (rmac a b c d) (newline))
<em class="gray">abcd</em>
</pre>

<p>The main built-in expansion is <b><em class=def id="readercond">reader-cond</em></b>.  The syntax is based on cond:
the car of each clause is evaluated (in the read-time context), and if it is not false,
the remainder of that clause is spliced into the code as if you had typed it from the start.
</p>

<pre class="indented">
&gt; '(1 2 (reader-cond ((> 1 0) 3) (else 4)) 5 6)
<em class="gray">(1 2 3 5 6)</em>
&gt; ((reader-cond ((> 1 0) list 1 2) (else cons)) 5 6)
<em class="gray">(1 2 5 6)</em>
</pre>


<!-- from kanren
(define-syntax conj*
  (syntax-rules ()
    ((conj*) succeed)
    ((conj* g) g)
    ((conj* g gs ...)
      (conj g (lambda (s) ((conj* gs ...) s))))))

is the same (in that context) as:

(define-macro (conj* . args)
  (if (null? args)
      succeed
      (if (null? (cdr args))
	  (car args)
	  `(conj ,(car args)
		 (lambda (s) ((conj* ,@(cdr args)) s))))))
-->


<div class="separator"></div>
<p id="profiling">If you build s7 with the flag -DWITH_PROFILE=1, s7 accumulates profiling data
in the (*s7* 'profile-info) hash-table.  The line number reported sometimes points to the end of the form.
Also, do loops and other iterations are sometimes highly optimized, so only the outer form is counted.
profile.scm shows one way to sort and display this data. To clear the counts, <code>(fill! (*s7* 'profile-info) #f)</code>.
</p>


<div class="separator"></div>
<p id="s7env"><b>*s7*</b> is an environment that gives access to some of s7's internal
state: 
</p>
<pre class="indented">
print-length                  number of elements of a sequence to print
max-string-length             max size arg to make-string and read-string
max-list-length               max size arg to make-list
max-vector-length             max size arg to make-vector and make-hash-table
max-vector-dimensions         make-vector dimensions limit
default-hash-table-length     default size for make-hash-table (8, tables resize as needed)
initial-string-port-length    128, initial size of a string port's buffer
history-size                  eval history buffer size if s7 built WITH_HISTORY=1
profile-info                  profiling data if s7 built WITH_PROFILE=1

default-rationalize-error     1e-12 (settable)
morally-equal-float-epsilon   1e-15 (settable)
hash-table-float-epsilon      1e-12 (settable, but currently limited to less than 1e-3).
bignum-precision              bits for bignum floats (128)
float-format-precision        digits to print for floats (16)
default-random-state          the default arg for random (settable)

cpu-time                      run time so far
file-names                    currently loaded files (a list)

safety                        0
undefined-identifier-warnings #f 

catches                       a list of the currently active catch tags
exits                         a list of active call-with-exit exit functions
c-types                       a list of c-object type names (from s7_new_type, etc)
input-ports, output-ports, strings, gensyms, vectors, hash-tables, continuations

stack-top                     current stack location
stack-size                    current stack size
max-stack-size                max stack size (settable)
stack                         the current stack entries
stacktrace-defaults           stacktrace formatting info for error handler

symbol-table                  a vector
symbol-table-locked?          #f (if #t, no new symbols can be added to the symbol table)
rootlet-size                  the number of globals

heap-size                     total cells currently available
free-heap-size                the number of currently unused cells
gc-freed                      number of cells freed by the last GC pass
gc-protected-objects          vector of the objects permanently protected from the GC
gc-stats                      #f (#t turns on printout of the GC activity)
memory-usage                  a description of current memory allocations
</pre>

<p>
Use the standard environment syntax to access these fields:
<code>(*s7* 'stack-top)</code>.  stuff.scm has the function
*s7*-&gt;list that returns most of these fields in a list.
</p>

<p>
Set (*s7* 'safety) to 2 or higher
to turn off optimization.
</p>

<p>stacktrace-defaults is a list of four integers and a boolean that tell the error
handler how to format stacktrace information.  The four integers are: 
how many frames to display, 
how many columns are devoted to code display,
how many columns are available for a line of data,
and where to place comments.
The boolean sets whether the entire output should be displayed as a comment.
The defaults are '(3 45 80 45 #t).
</p>

<pre class="indented">
(<em class=def id="cobject">c-object?</em> obj)
(<em class=def id="cpointer">c-pointer?</em> obj)
(<em class=def id="cpoint">c-pointer</em> int)
</pre>

<p>
You can wrap up raw C pointers and
pass them around in s7 code. The function c-pointer returns a wrapped pointer,
and c-pointer? returns #t if passed one.  <code>(define NULL (c-pointer 0))</code>.
c-object? returns the object's type tag if passed such an object (otherwise #f of course).  This tag is also the position
of the object's type in the (*s7* 'c-types) list. 
(*s7* 'c-types) returns a list of the types created by s7_new_type_x and friends.
</p>


<div class="separator"></div>

<blockquote>

<div class="indented">
<p id="s7vsr5rs">Some other differences from r5rs:
</p>

<ul>
<li>no force or delay (see <a href="#r7rs">below</a>).
<li>no syntax-rules or any of its friends.
<li>no scheme-report-environment, null-environment, or interaction-environment (use curlet).
<li>no transcript-on or transcript-off.
<li>begin returns the value of the last form; it can contain both definitions and other statements.
<li>#&lt;unspecified&gt;, #&lt;eof&gt;, and #&lt;undefined&gt; are first-class objects.
<li>for-each and map accept different length arguments; the operation stops when any argument reaches its end.
<li>for-each and map accept any applicable object as the first argument, and any sequence or iterator as a trailing argument.
<li>letrec*, but without conviction.
<li>set! and *-set! return the new value (modulo symbol-access), not #&lt;unspecified&gt;.
<li>define and its friends return the new value.
<li>port-closed? 
<li>list? means "pair or null", proper-list? is r5rs list?, float? means "real and not rational", sequence? = length.
<!-- a vector can be a member of itself, and yet vector? returns #t, why is list? different; we even call it a circular list! -->
<!-- <li>current-input-port, current-output-port, and current-error-port have setters (but I may remove these) -->
<li>the default IO ports are named *stdin*, *stdout*, and *stderr*.
<li>#f as an output port means nothing is output (#f is /dev/null, I guess).
<li>member and assoc accept an optional third argument, the comparison function (equal? is the default).
<li>case accepts =&gt; much like cond (the function argument is the selector).
<li>if WITH_SYSTEM_EXTRAS is 1, the following are built-in: directory?, file-exists?, delete-file, system, directory-&gt;list, getenv.
<li>s7 is case sensitive.
<li>when and unless (for r7rs), returning the value of the last form.
<li>the "d", "f", "s", and "l" exponent markers are not supported by default (use "e", "E", or "@").
<li>quasiquoted vector constants are not supported (use the normal list expansions wrapped in list-&gt;vector).
</ul>

<p>In s7 if a built-in function like gcd is referred to in a function
body, the optimizer is free to replace it with #_function.  That is, <code>(gcd ...)</code> can be changed
to <code>(#_gcd ...)</code> at s7's whim, if gcd has its original value at the time the optimizer
sees the expression using it.  A subsequent <code>(set! gcd +)</code> does not affect this optimized call.
I think I could wave my hands and mumble about "aggressive lexical scope" or something, but actually the
choice here is that speed trumps that ol' hobgoblin consistency.  If you want to change gcd to +, do it before
loading code that calls gcd.  
I think most Schemes handle macros this way: the macro call is replaced by its expansion using its current
definition, and a later redefinition does not affect earlier uses.
</p>

<!-- another case:  (with-let (inlet '+ -) (+ 2 3)) -> 5 -->
<!-- also, (eq? (if #f #f) (apply values ())) is #t, but memq and assq don't treat them as equal -->

</div>

<!-- null-environment does not fit very well; how to shadow earlier define-constant for example
     one of which is unlet; perhaps make a special environment at init time? 
     the code for that is save_null_environment, but it is untested and commented out.
-->

<div class="indented">

<p>Here are some changes I'd make to s7 if I didn't care about compatibility with other Schemes:
</p>

<ul>
<li>remove the exact/inexact distinction including #i and #e
<li>remove call-with-values and its friends
<li>remove char-ready?
<li>change eof-object? to eof? or just omit it (you can use eq? #&lt;eof&gt;)
<li>change make-rectangular to complex, and remove make-polar.
<li>remove unquote (the name, not the functionality).
<li>remove cond-expand.
<li>remove *-ci functions
<li>remove #d
</ul>

<p>(most of these are removed if you set the compiler flag WITH_PURE_S7), and perhaps:
</p>

<ul>
<li>remove even? and odd?, gcd and lcm.
<li>remove string-length and vector-length.
<li>remove list-ref|set!, string-ref|set!, vector-ref|set!, hash-table-ref|set!, set-car!|cdr!, and set-current-output|input|error-port.
<li>change file-exists? to file? (or omit it and assume the use of libc.scm &mdash; why reinvent the wheel?).
<li>remove all the conversion and copy functions like vector-&gt;list and vector-copy (use copy or map).
<li>add typeq? and perhaps make null? generic (see stuff.scm).
<li>change string-&gt;symbol to symbol (what to do with symbol-&gt;string in that case?)
<li>change with-output-to-* and with-input-from-* to omit the pointless lambda.
<li>remove the with-* IO functions (e.g. with-input-from-string), keeping the call-with-* versions (call-with-input-string).
<li>remove assq, assv, memq, and memv (these are pointless now that assoc and member can be passed eq? and eqv?).
</ul>

<p>There are several less-than-ideal names.  Perhaps s7 should use *pi*, *most-negative-fixnum*,
*most-positive-fixnum* (*fixmost*?) so that all the built-in variables and constants have the
same kind of name (or +pi+ to show it is a constant?).  
Perhaps object-&gt;string should be -&gt;string.
get-output-string should be current-output-string. write-char behaves like display, not write.
provided? should be feature? or *features* should be *provisions*.
</p>

<p>
The name "cond-expand" is bad &mdash; we're not expanding anything, and the macro's point is to make it easy to
operate with the *features* list; perhaps "cond-provided"?  Not only is cond-expand poorly named, but the whole
idea is clumsy.  Take libgsl.scm; different versions of the GSL library have different functions.  We need to know
when we're building the FFI what GSL version we're dealing with.  It would be nuts to start pushing and checking dozens
of library version symbols when all we actually want is <code>(&gt; version 23.19)</code>. (We are also politely
trying to ignore the ridiculous little language built into cond-expand; are we not running Scheme?).
In place of cond-expand, s7 uses <a href="#readercond">reader-cond</a>,
so the read-time decision involves normal evaluation.
</p>

<p>Then there's the case case: a case clause without a result appears to be an error in r7rs.
But the notation used to indicate that is the same as that used for begin.
So if we allow <code>(begin)</code>, we should allow case clauses to have no explicit result.
Currently in s7, the case case is an error and <code>(begin)</code> returns nil, which strikes me
as, *horrors*, inconsistent.  But in cond,
the "implicit progn" (in CL terminology) includes the test expression, so a clause without a result returns
the test result (if true of course).  In the case case, a similar approach might return
the selector value.  So should <code>(case x ((0 1)))</code> be an error, or equivalent
to <code>(case x ((0 1) (begin)))</code> as in CL, or (my favorite) <code>(case x ((0 1) => values))</code>.
If the latter, an empty else statement would also return the selector since it would be the
same as <code>(else => values)</code>.
</p>


<p>
Better ideas are always welcome!
</p>

<p>Here are the built-in s7 variables:
</p>
<ul>
<li>*features*        ; a list of symbols
<li>*libraries*       ; a list of (filename . let) pairs
<li>*load-path*       ; a list of directories
<li>*cload-directory* ; directory for cload output
<li>*autoload*        ; autoload info
<li>*#readers*        ; a list of (char . handler) pairs
</ul>

<p>And the built-in constants:
</p>
<ul>
<li>pi
<li>*stdin* *stdout* *stderr*
<li>*s7*
<li>nan.0 -nan.0 inf.0 -inf.0 (what crappy names! nan.0 is an inexact integer that is not a number?)
<li>most-positive-fixnum most-negative-fixnum
<li>*unbound-variable-hook* *missing-close-paren-hook* *load-hook* *error-hook* *read-error-hook*
</ul>

<p>__func__ is the name (or name and location) of the function currently being called, as in C.
</p>

<p>Currently WITH_PURE_S7:
</p>
<ul>
<li>places 'pure-s7 in *features*
<li>omits char-ready, char-ci*, string-ci*
<li>omits string-copy, string-fill!, vector-fill!, vector-append
<li>omits list-&gt;string, list-&gt;vector, string-&gt;list, vector-&gt;list, let-&gt;list
<li>omits string-length and vector-length
<li>omits cond-expand, multiple-values-bind|set!, call-with-values, defmacro(*)
<li>omits unquote (the name)
<li>omits d/f/s/l exponents
<li>omits make-polar and make-rectangular (use complex)
<li>omits integer-length, exact?, inexact?, exact-&gt;inexact, inexact-&gt;exact, #i and #e
<li>omits set-current-output-port and set-current-input-port
</ul>

</div>


<details>
<summary class="indented">quotes</summary>
<div class="indented">

<p>Schemes vary in their treatment of ().  s7 considers it a constant that evaluates to itself,
so you don't need to quote it.  <code>(eq? () '())</code> is #t.
This is consistent with, for example,
<code>(eq? #f '#f)</code> which is also #t.
The standard says "the empty list is a special object of its own type", so surely either choice is
acceptable in that regard.  One place where the quote matters is in a case statement; the selector is
evaluated but the key is not:
</p>

<pre class="indented">
&gt; (case '() ((()) 2) (else 1))
<em class="gray">2</em>
&gt; (case '() (('()) 2) (else 1)) ; (eqv? '() ''()) is #f
<em class="gray">1</em>
;;; which parallels #f (or a number such as 2 etc):
&gt; (case '#f ((#f) 2) (else 1))
<em class="gray">2</em>
&gt; (case '#f (('#f) 2) (else 1)) ; (eqv? '#f ''#f) is #f
<em class="gray">1</em>
</pre>

<p>Similarly, vector constants do not have to be quoted.  A list constant is quoted
to keep it from being evaluated, but
#(1 2 3) is as unproblematic as "123" or 123.
</p>

<!-- there's another sense in which '() is a constant: you can't apply it to anything. ('() 0) -> error
-->

<p>These examples bring up another odd corner of scheme: else.  In <code>(cond (else 1))</code>
the 'else is evaluated (like any cond test), so its value might be #f; in <code>(case 0 (else 1))</code>
it is not evaluated (like any case key), so it's just a symbol.  
Since symbol accessors are local in s7, 
someone can <code>(let ((else #f)) (cond (else 1)))</code> even if we protect the rootlet 'else.
Of course, in scheme this kind of trouble is pervasive, so rather than make 'else a constant
I think the best path is to use unlet:
<code>(let ((else #f)) (cond (#_else 1)))</code>.  This is 1 (not ()) because the initial value of 'else
can't be changed.
</p>
</div>
</details>


<details>
<summary class="indented">mutable constants</summary>
<div class="indented">

<p>How should s7 treat this:
<code>(string-set! "hiho" 1 #\z)</code>, or 
<code>(vector-set! #(1 2 3) 1 32)</code>, or
<code>(list-set! '(1 2 3) 1 32)</code>?
Originally, in s7, the first two were errors, and the third was allowed, which doesn't make much sense.
Guile and Common Lisp accept all three, but that leads to weird cases where we can reach
into a function's body:
</p>

<pre class="indented">
&gt; (let ((x (lambda () '(1 2 3)))) (list-set! (x) 1 32) (x))
<em class="gray">(1 32 3)</em> ; s7, Guile
&gt; (flet ((x () '(1 2 3))) (setf (nth 1 (x)) 32) (x))
<em class="gray">(1 32 3)</em> ; Clisp
&gt; (let ((x (lambda () (list 1 2 3)))) (list-set! (x) 1 32) (x))
<em class="gray">(1 2 3)</em>
</pre>

<p>
But it's possible to reach into a function's closure, even when the
closed-over thing is a constant:
</p>

<pre class="indented">
&gt; (flet ((x () '(1 2 3))) (setf (nth 1 (x)) 32) (x))
<em class="gray">(1 32 3)</em>
&gt; (let ((xx (let ((x '(1 2 3))) (lambda () x)))) (list-set! (xx) 1 32) (xx))
<em class="gray">(1 32 3)</em>
&gt; (let ((xx (let ((x (list 1 2 3))) (lambda () x)))) (list-set! (xx) 1 32) (xx))
<em class="gray">(1 32 3)</em>
</pre>

<p>
And it's possible to reach into a constant list via list-set! (or set-car! of course):
</p>

<pre class="indented">
&gt; (let* ((x '(1 2)) (y (list x)) (z (car y))) (list-set! z 1 32) (list x y z))
<em class="gray">((1 32) ((1 32)) (1 32))</em>
</pre>

<p>
It would be a programmer's nightmare to have to keep track of which piece of a list is
constant, and an implementor's nightmare to copy every list.  set! in all its forms is
used for its side-effects, so why should we try to put a fence around them?
If we flush "immutable constant" because it is a ham-fisted, whack-it-with-a-shovel approach,
the only real problem I can see is symbol-&gt;string.  In CL, this is explicitly an error:
</p>

<pre class="indented">
&gt; (setf (elt (symbol-name 'xyz) 1) #\X)
<em class="gray">*** - Attempt to modify a read-only string: "XYZ"</em>
</pre>

<p>And in Guile:
</p>

<pre class="indented">
&gt; (string-set! (symbol-&gt;string 'symbol-&gt;string) 1 #\X)
<em class="gray">ERROR: string is read-only: "symbol-&gt;string"</em>
</pre>

<p>So both have a notion of immutable strings.  
I wonder what other Scheme programmers (not implementors!) want in this situation.
Currently, there are no immutable list, string, or vector constants, and
symbol-&gt;string
returns a copy of the string.
copy can protect some values.  Or combine object-&gt;string and string-&gt;symbol:
</p>

<pre class="indented">
&gt; (let ((x (lambda () (copy "hiho")))) (string-set! (x) 1 #\x) (x))
<em class="gray">"hiho"</em>
&gt; (let ((x (string-&gt;symbol "hiho"))) (string-set! (symbol-&gt;string x) 1 #\x) (symbol-&gt;string x))
<em class="gray">"hiho"</em>
&gt; (define (immutable obj) (string-&gt;symbol (object-&gt;string obj :readable)))
<em class="gray">immutable</em>
&gt; (define (symbol-&gt;object sym) (eval-string (symbol-&gt;string sym)))
<em class="gray">symbol-&gt;object</em>
&gt; (symbol-&gt;object (immutable (list 1 2 3)))
<em class="gray">(1 2 3)</em>
</pre>

<p>s7 normally tries to optimize garbage collection by
removing some list constants from the heap.  If you later
set a member of it to something that needs GC protection, nobody in the heap points to it, so
it is GC'd.  Here is an example:
</p>

<pre class="indented">
(define (bad-idea)
  (let* ((lst '(1 2 3))
         (result (list-ref lst 1)))
   (list-set! lst 1 (* 2.0 16.6))
   (gc)
   result))
</pre>

<p>Put this is a file, load it into the interpreter, then call <code>(bad-idea)</code> a
few times.  You can turn off the optimization in question by setting the variable <code>(*s7* 'safety)</code>
to 1.  <code>(*s7* 'safety)</code> defaults to 0.  
</p>

<p>A similar problem arises when you want to walk a function's source or reuse a piece of
code directly.  When the function is evaluated, the optimizer changes the program source
to speed up subsequent evaluation.  This annotation process means that nothing in that
source is what it appears to be, so a tree walker will be confused, and if you copy that
source and try to insert it into some other program source, the existing annotations
will not fit the new context.  In both cases, you can get a clean version of the code
by copying it with :readable as the second argument to copy.  There is an example
in lint.scm, and in the snd file tools/tgen.scm.
</p>

</div>
</details>


<details id="circle">
<summary class="indented">circular lists</summary>
<div class="indented">

<p>s7 handles circular lists and vectors and dotted lists with its customary aplomb.  
You can pass them to memq, or print them, for example; you can even evaluate them.  
The print syntax is borrowed from CL:
</p>

<pre class="indented">
&gt; (let ((lst (list 1 2 3))) 
    (set! (cdr (cdr (cdr lst))) lst) 
    lst)
<em class="gray">#1=(1 2 3 . #1#)</em>
&gt; (let* ((x (cons 1 2)) 
         (y (cons 3 x))) 
    (list x y))
<em class="gray">(#1=(1 . 2) (3 . #1#))</em>
</pre>

<p id="circularlistreader">
But should this syntax be readable as well?  I'm inclined to say no because
then it is part of the language, and it doesn't look like the rest of the language.
(I think it's kind of ugly).  Perhaps we could implement it via *#readers*:
</p>

<pre>
(define circular-list-reader
  (let ((known-vals #f)
	(top-n -1))
    (lambda (str)

      (define (replace-syms lst)
	;; walk through the new list, replacing our special keywords 
        ;;   with the associated locations

	(define (replace-sym tree getter)
	  (if (keyword? (getter tree))
	      (let ((n (string-&gt;number (symbol-&gt;string (keyword-&gt;symbol (getter tree))))))
		(if (integer? n)
		    (let ((lst (assoc n known-vals)))
		      (if lst
			  (set! (getter tree) (cdr lst))
			  (format *stderr* "#~D# is not defined~%" n)))))))

	(let walk-tree ((tree (cdr lst)))
	  (if (pair? tree)
	      (begin
		(if (pair? (car tree)) (walk-tree (car tree)) (replace-sym tree car))
		(if (pair? (cdr tree)) (walk-tree (cdr tree)) (replace-sym tree cdr))))
	  tree))

      ;; str is whatever followed the #, first char is a digit
      (let* ((len (length str))
	     (last-char (str (- len 1))))
	(and (memv last-char '(#\= #\#))             ; is it #n= or #n#?
	    (let ((n (string-&gt;number (substring str 0 (- len 1)))))
	      (and (integer? n)
		  (begin
		    (if (not known-vals)            ; save n so we know when we're done
			(begin
			  (set! known-vals ())
			  (set! top-n n))) 

		    (if (char=? last-char #\=)      ; #n=
			(and (eqv? (peek-char) #\() ; eqv? since peek-char can return #&lt;eof&gt;
			    (let ((cur-val (assoc n known-vals)))
			      ;; associate the number and the list it points to
			      ;;    if cur-val, perhaps complain? (#n# redefined)
			      (let ((lst (catch #t 
					   read
					   (lambda args             ; a read error
					     (set! known-vals #f)   ;   so clear our state
					     (apply throw args))))) ;   and pass the error on up
				(if cur-val
                                    (set! (cdr cur-val) lst)
				    (set! known-vals 
					  (cons (set! cur-val (cons n lst)) known-vals))))

			      (if (= n top-n)            ; replace our special keywords
				  (let ((result (replace-syms cur-val)))
				    (set! known-vals #f) ; '#1=(#+gsl #1#) -&gt; '(:1)!
				    result)
				  (cdr cur-val))))
			                         ; #n=&lt;not a list&gt;?
			;; else it's #n# &mdash; set a marker for now since we may not 
			;;   have its associated value yet.  We use a symbol name that 
                        ;;   string-&gt;number accepts.
			(symbol-&gt;keyword 
                          (symbol (number-&gt;string n) (string #\null) " "))))))
		                                 ; #n&lt;not an integer&gt;?
	    )))))                                ; #n&lt;something else&gt;?

(do ((i 0 (+ i 1)))
    ((= i 10))
  ;; load up all the #n cases
  (set! *#readers* 
    (cons (cons (integer-&gt;char (+ i (char-&gt;integer #\0))) circular-list-reader)
          *#readers*)))
<!-- ) -->
&gt; '#1=(1 2 . #1#)
<em class="gray">#1=(1 2 . #1#)</em>
&gt; '#1=(1 #2=(2 . #2#) . #1#)
<em class="gray">#2=(1 #1=(2 . #1#) . #2#)</em>
</pre>

<p>And of course, we can treat these as labels:
</p>

<pre class="indented">
(let ((ctr 0)) #1=(begin (format () "~D " ctr) (set! ctr (+ ctr 1)) (if (&lt; ctr 4) #1# (newline))))
</pre>

<p>which prints "0 1 2 3" and a newline.
</p>

<br>


<p>Length returns +inf.0 if passed a circular list, and returns a negative
number if passed a dotted list.  In the dotted case, the absolute value of the length is the list length not counting
the final cdr.  <code>(define (circular? lst) (infinite? (length lst)))</code>.
</p>

<p>
<em class=def id="cyclicsequences">cyclic-sequences</em> returns a list of the cyclic
sequences in its argument, or nil.
<code>(define (cyclic? obj) (pair? (cyclic-sequences obj)))</code>.
</p>

<p>Here's an amusing use of circular lists:
</p>

<pre class="indented">
(define (for-each-permutation func vals)
  ;; apply func to every permutation of vals: 
  ;;   (for-each-permutation (lambda args (format () "~{~A~^ ~}~%" args)) '(1 2 3))
  (define (pinner cur nvals len)
    (if (= len 1)
        (apply func (car nvals) cur)
        (do ((i 0 (+ i 1)))                       ; I suppose a named let would be more Schemish
            ((= i len))
          (let ((start nvals))
            (set! nvals (cdr nvals))
            (let ((cur1 (cons (car nvals) cur)))  ; add (car nvals) to our arg list
              (set! (cdr start) (cdr nvals))      ; splice out that element and 
              (pinner cur1 (cdr start) (- len 1)) ;   pass a smaller circle on down, "wheels within wheels"
              (set! (cdr start) nvals))))))       ; restore original circle
  (let ((len (length vals)))
    (set-cdr! (list-tail vals (- len 1)) vals)    ; make vals into a circle
    (pinner () vals len)
    (set-cdr! (list-tail vals (- len 1)) ())))    ; restore its original shape
</pre>
</div>
</details>


<details>
<summary class="indented">unprintable symbols</summary>
<div class="indented">

<p>s7 and Snd use "*" in a variable name, *features* for example, to indicate
that the variable is predefined.  It may occur unprotected in a macro, for
example.  The "*" doesn't mean that the variable is special in the CL sense of dynamic scope,
but some clear marker is needed for a global variable so that the programmer
doesn't accidentally step on it.  
</p>

<p>Although a variable name's first character is more restricted, currently
only #\null, #\newline, #\tab, #\space, #\), #\(, #\", and #\; can't
occur within the name.  I did not originally include double-quote in this set, so wild stuff like
<code>(let ((nam""e 1)) nam""e)</code>
would work, but that means that <code>'(1 ."hi")</code> is parsed as a 1 and the
symbol <code>."hi"</code>, and <code>(string-set! x"hi")</code> is an error.
The first character should not be #\#, #\', #\`, #\,, #\:, or any of those mentioned above,
and some characters can't occur by themselves.  For example, "." is not a legal variable
name, but ".." is. 
These weird symbols have to be printed sometimes:
</p>

<pre class="indented">
&gt; (list 1 (string-&gt;symbol (string #\; #\" #\\)) 2)
<em class="gray">(1 ;"\ 2)</em>            <!-- " -->
&gt; (list 1 (string-&gt;symbol (string #\.)) 2)
<em class="gray">(1 . 2)</em>
</pre>

<p>which is a mess.  Guile prints the first as <code>(1 #{\;\"\\}# 2)</code>.
In CL and some Schemes:
</p>

<pre class="indented">
[1]&gt; (list 1 (intern (coerce (list #\; #\" #\\) 'string)) 2) ; thanks to Rob Warnock
<em class="gray">(1 |;"\\| 2)</em>        <!-- " -->
[2]&gt; (equalp 'A '|A|) ; in CL case matters here
<em class="gray">T</em>
</pre>

<p>This is clean, and has the weight of tradition behind it, but 
I think I'll use "symbol" instead:
</p>

<pre class="indented">
&gt; (list 1 (string-&gt;symbol (string #\; #\" #\\)) 2)
<em class="gray">(1 (symbol ";\"\\") 2)</em>       <!-- " -->
</pre>

<p>
This output is readable, and does not eat up perfectly good
characters like vertical bar, but it means we can't easily use
variable names like "| e t c |".  We could allow a name to
contain any characters if it starts and ends with "|",
but then one vertical bar is trouble.  (The symbol function
actually accepts any number of string arguments which it concatenates
to form the new symbol name).
</p>

<p>
These symbols are not just an optimization of string comparison:
</p>

<pre class="indented">
&gt; (define-macro (hi a) 
  (let ((funny-name (string-&gt;symbol ";")))
    `(let ((,funny-name ,a)) (+ 1 ,funny-name))))
<em class="gray">hi</em>
&gt; (hi 2)
<em class="gray">3</em>
&gt; (macroexpand (hi 2))
<em class="gray">(let ((; 2)) (+ 1 ;))</em>    ; for a good time, try (string #\")

&gt; (define-macro (hi a) 
  (let ((funny-name (string-&gt;symbol "| e t c |")))
    `(let ((,funny-name ,a)) (+ 1 ,funny-name))))
<em class="gray">hi</em>
&gt; (hi 2)
<em class="gray">3</em>
&gt; (macroexpand (hi 2))
<em class="gray">(let ((| e t c | 2)) (+ 1 | e t c |))</em>
&gt; (let ((funny-name (string-&gt;symbol "| e t c |"))) ; now use it as a keyword arg to a function
    (apply define* `((func (,funny-name 32)) (+ ,funny-name 1)))
    ;; (procedure-source func) is (lambda* ((| e t c | 32)) (+ | e t c | 1))
    (apply func (list (symbol-&gt;keyword funny-name) 2)))
<em class="gray">3</em>
</pre>

<p>I hope that makes you as happy as it makes me!
</p>
</div>
</details>


<div class="indented">

<p id="legolambda">The built-in syntactic forms, such as "begin", are almost first-class citizens.
</p>

<pre class="indented">
&gt; (let ((progn begin)) 
    (progn 
      (define x 1) 
      (set! x 3) 
      (+ x 4)))
<em class="gray">7</em>
&gt; (let ((function lambda)) 
    ((function (a b) (list a b)) 3 4))
<em class="gray">(3 4)</em>
&gt; (apply begin '((define x 3) (+ x 2)))
<em class="gray">5</em>
&gt; ((lambda (n) (apply n '(((x 1)) (+ x 2)))) let)
<em class="gray">3</em>

(define-macro (symbol-set! var val) ; like CL's set
  `(apply set! ,var ',val ()))      ; trailing nil is just to make apply happy &mdash; apply*?

(define-macro (progv vars vals . body)
 `(apply (apply lambda ,vars ',body) ,vals))

&gt; (let ((s '(one two)) (v '(1 2))) (progv s v (+ one two)))
<em class="gray">3</em>
</pre>

<p>We can snap together program fragments ("look Ma, no macros!"):
</p>

<pre class="indented">
(let* ((x 3) 
       (arg '(x)) 
       (body `((+ ,x x 1)))) 
  ((apply lambda arg body) 12)) ; "legolambda"?

(define (engulph form)
  (let ((body `(let ((L ()))
		 (do ((i 0 (+ i 1)))
		     ((= i 10) (reverse L))
		   (set! L (cons ,form L))))))
    (define function (apply lambda () (list (copy body :readable))))
    (function)))

(let ()
  (define (hi a) (+ a x))
  ((apply let '((x 32)) (list (procedure-source hi))) 12)) ; one function, many closures?

(let ((ctr -1))  ; (enum zero one two) but without using a macro
  (apply begin 
    (map (lambda (symbol) 
           (set! ctr (+ ctr 1)) 
           (list 'define symbol ctr)) ; e.g. '(define zero 0) 
         '(zero one two)))
  (+ zero one two))
</pre>

<p>But there's a prettier way to implement enum ("transparent-for-each"):
</p>

<pre class="indented">
&gt; (define-macro (enum . args)
    `(for-each define ',args (iota (length ',args))))
<em class="gray">enum</em>
&gt; (enum a b c) 
<em class="gray">#&lt;unspecified&gt;</em>
&gt; b
<em class="gray">1</em>
</pre>


<p><code>(apply define ...)</code> is similar to CL's set.
</p>

<pre class="indented">
&gt; ((apply define-macro '((m a) `(+ 1 ,a))) 3)
<em class="gray">4</em>
&gt; ((apply define '((hi a) (+ a 1))) 3)
<em class="gray">4</em>
</pre>

<p>This gives us a way to make anonymous macros, just as lambda returns an anonymous function:
</p>

<pre class="indented">
&gt; (define-macro (mu args . body)
  `(apply define-macro '((,(gensym) ,@args) ,@body)))
<em class="gray">mu</em>
&gt; ((mu (a) `(+ 1 ,a)) 3)
<em class="gray">4</em>
&gt; (define-macro (glambda args) ; returns an anonymous macro that will return a function given a body
    `(define-macro (,(gensym) . body) 
         `(lambda ,',args ,@body)))
<em class="gray">glambda</em>
&gt; (let ((gf (glambda (a b))))  ; gf is now ready for any body that involves arguments 'a and 'b
    ((gf (+ a b)) 1 2))        ; apply (lambda (a b) (+ a b)) to '(1 2)
<em class="gray">3</em>
</pre>

<p>catch, dynamic-wind, and many of the other functions that take function
arguments in standard Scheme, accept macros in s7.
</p>

<p>Apply let is very similar to eval:
</p>
<pre>
&gt; (apply let '((a 2) (b 3)) '((+ a b)))
<em class="gray">5</em>
&gt; (eval '(+ a b) (inlet 'a 2 'b 3))
<em class="gray">5</em>
&gt; ((apply lambda '(a b) '((+ a b))) 2 3)
<em class="gray">5</em>
&gt; (apply let '((a 2) (b 3)) '((list + a b))) ; a -&gt; 2, b -&gt; 3
<em class="gray">(+ 2 3)</em>
</pre>
<p>The redundant-looking double lists are for apply's benefit.  We could
use a trailing null instead (mimicking apply* in some ancient lisps):
</p>
<pre>
&gt; (apply let '((a 2) (b 3)) '(list + a b) ())
<em class="gray">(+ 2 3)</em>
</pre>

<p>
Currently, you can't set! a built-in syntactic keyword to some new value:
<code>(set! if 3)</code>.
I hope this kind of thing is not actually very useful, but let me
know if you need it.  The issue is purely one of speed.
</p>

<p>Speaking of speed... It is widely believed
that a Scheme with first class everything can't hope to compete with any
"real" Scheme.  Humph I say.  Take this little example (which is not 
so misleading that I feel guilty about it):
</p>
<pre class="indented">
(define (do-loop n)
  (do ((i 0 (+ i 1)))
      ((= i n))
    (if (zero? (modulo i 1000))
	(display ".")))
  (newline))

(for-each
 (lambda (n) (do-loop n))
 (list 1000 1000000 10000000))
</pre>

<p>In s7, that takes 0.24 seconds on my home machine.  In tinyScheme, from
whence we sprang, it takes 85 seconds.  In the chicken interpreter, 5.3
seconds, and after compilation (using -O2) of the chicken compiler output,
0.75 seconds.  So, s7 is comparable to chicken in speed, even though chicken
is compiling to C. I think Guile 2.0.9 takes about 1 second.
The equivalent in CL:
clisp interpreted 9.3 seconds, compiled 0.85 seconds; sbcl 0.21 seconds.
</p>
</div>


<div class="indented">

<p>In s7, there is only one kind of begin statement,
and it can contain both definitions and expressions.  These are evaluated in the order
in which they occur, and in the environment at the point of the evaluation.  I think
of it as being a little REPL.  begin does not introduce a new frame in
the current environment, so defines happen in the enclosing environment.
</p>
</div>


<div class="indented">

<p id="r7rs">The r7rs compatibility code is in r7rs.scm. I used to include it here, but
as r7rs grew, this section got too large.  In general, all the conversion routines in
r7rs are handled in s7 via generic functions, records are classes, byte-vectors are
strings, and so on. 
</p>
</div>


<details>
<summary class="indented">threads</summary>
<div class="indented">

<p>s7 originally had multithreading support, but I removed it in August, 2011.
It turned out to be less useful than I hoped,
mainly because s7 threads shared the heap and therefore had to coordinate
all cell allocations.  It was faster and simpler to use multiple
processes each running a separate s7 interpreter, rather than one s7
running multiple s7 threads.  In CLM, there was also contention for access
to the output stream.  In GUI-related situations,
threads were not useful mainly because the GUI toolkits are not thread safe.
Last but not least, the effort to make the non-threaded
s7 faster messed up parts of the threaded version.  Rather than
waste a lot of time fixing this, I chose to flush multithreading.
Here's a very simple example of using an s7 interpreter per thread:
</p>

<pre class="indented">
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;pthread.h&gt;
#include "s7.h"

typedef struct {
  s7_scheme *sc;
  s7_pointer func;
  pthread_t *thread;
} thred;

static void *run_thread(void *obj)
{
  thred *f = (thred *)obj;
  return((void *)s7_call(f-&gt;sc, f-&gt;func, s7_nil(f-&gt;sc)));
}

static thred *make_thread(s7_function func)
{
  thred *f;
  f = (thred *)malloc(sizeof(thred));
  f-&gt;sc = s7_init();
  f-&gt;func = s7_make_function(f-&gt;sc, "a-test", func, 0, 0, false, "a test");
  f-&gt;thread = (pthread_t *)malloc(sizeof(pthread_t));
  pthread_create(f-&gt;thread, NULL, run_thread, (void *)f);
  return(f);
}

static s7_pointer a_test(s7_scheme *sc, s7_pointer args)
{
  fprintf(stderr, "I am %p\n", sc);
  /* do something time-consuming... */
  return(args);
}

int main(int argc, char **argv)
{
  thred *f1, *f2;
  f1 = make_thread(a_test);
  f2 = make_thread(a_test);

  pthread_join(*(f1-&gt;thread), NULL);
  pthread_join(*(f2-&gt;thread), NULL);
}

/* build s7 with -DWITH_THREADS, then
 * gcc -o repl repl.c s7.o -g3 -Wl,-export-dynamic -lpthread -lm -I. -ldl 
 */
</pre>

<p>Unfortunately, there's no way yet to
free all the resources s7_init allocates (the heap, stack, etc).
</p>

</div>
</details>

<div class="indented">

<p>"Life", a poem.
</p>

<pre class="indented">
(+(*(+))(*)(+(+)(+)(*)))
(((((lambda () (lambda () (lambda () (lambda () 1))))))))
(+ (((lambda () values)) 1 2 3))
(map apply (list map) (list map) (list (list *)) '((((1 2)) ((3 4 5)))))
(do ((do do do)) (do do do))
(*(*)(*) (+)(+) 1)
</pre>

</div>

</blockquote>
<br><br>


<div class="topheader" id="FFIexamples">FFI examples</div>

<p>s7 exists only to serve as an extension of some other application, so
it is primarily a foreign function interface.  s7.h has lots of comments about the individual
functions.  Here I'll collect some complete examples.  s7.c depends on the following
compile-time flags:
</p>

<pre class="indented">
SIZEOF_VOID_P                  8 (default) or 4.
WITH_GMP                       1 if you want multiprecision arithmetic (requires gmp, mpfr, and mpc, default is 0)
HAVE_COMPLEX_NUMBERS           1 if your compiler supports complex numbers
HAVE_COMPLEX_TRIG              1 if your math library has complex versions of the trig functions
DISABLE_DEPRECATED             1 if you want to make sure you're not using any deprecated s7 stuff (default is 0)

WITH_IMMUTATBLE_UNQUOTE        1 if you want "unquote" omitted (default is 0)
WITH_EXTRA_EXPONENT_MARKERS    1 if you want "d", "f", "l", and "s" in addition to "e" as exponent markers (default is 0)
                                   if someone defends these exponent markers, ask him to read 1l11+11l1i
                                   (in 2 million lines of open-source Scheme, there is not one use of these silly things)
WITH_SYSTEM_EXTRAS             1 if you want some additional OS-related functions built-in (default is 0)
WITH_MAIN                      1 if you want s7.c to include a main program section that runs a REPL.
WITH_C_LOADER		       1 if you want to be able to load shared object files with load.
</pre>

<p>See the comment at the start of s7.c for more information about these switches.
s7.h defines the two main number types: s7_int and s7_double.
The examples that follow show:
</p>

<ul>
<li><a href="#repl">read-eval-print loop (and emacs)</a>
<li><a href="#defun">define a function with arguments and a returned value, and define a variable </a>
<li><a href="#defvar">call a Scheme function from C, and get/set Scheme variable values in C</a>
<li><a href="#juce">C++ and Juce</a>
<li><a href="#sndlib">load sndlib using the Xen functions and macros</a>
<li><a href="#pwstype">add a new Scheme type and a procedure with a setter</a>
<li><a href="#functionportexample">redirect display output to a C procedure</a>
<li><a href="#extendop">extend a built-in operator ("+" in this case)</a>
<li><a href="#definestar1">C-side define* (s7_define_function_star)</a>
<li><a href="#definemacro1">C-side define-macro (s7_define_macro)</a>
<li><a href="#definegeneric">define a generic function in C</a>
<li><a href="#signal">signal handling (C-C to break out of an infinite loop)</a>
<li><a href="#vector">direct multidimensional vector element access</a>
<li><a href="#notify">notification in C that a Scheme variable has been set!</a>
<li><a href="#namespace">Load C defined stuff into a separate namespace</a>
<li><a href="#Cerrors">Error handling in C</a>
<li><a href="#testhook">Hooks in C and Scheme</a>
<li><a href="#dload">Load a C module dynamically</a>
<li><a href="#gmpex">gmp and friends</a>
<li><a href="#glistener">glistener.c</a>
<li><a href="#gdb">gdb</a>
</ul>


<div class="header" id="repl"><h4>A simple listener</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include "s7.h"

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = <em class=red>s7_init</em>();                 /* initialize the interpreter */
  while (1)                       /* fire up a read-eval-print loop */
    {
      fprintf(stdout, "\n&gt; ");    /* prompt for input */
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                         /* evaluate the input and print the result */
	  sprintf(response, "(write %s)", buffer);
	  <em class=red>s7_eval_c_string</em>(s7, response); 
	}
    }
}

/* make mus-config.h (it can be empty), then
 *
 *   gcc -c s7.c -I.
 *   gcc -o repl repl.c s7.o -lm -I. -ldl
 *
 * run it:
 *
 *    repl
 *    &gt; (+ 1 2)
 *    <em class="gray">3</em>
 *    &gt; (define (add1 x) (+ 1 x))
 *    <em class="gray">add1</em>
 *    &gt; (add1 2)
 *    <em class="gray">3</em>
 *    &gt; (exit)
 *
 * for long-term happiness in linux use:
 *   gcc -o repl repl.c s7.o -Wl,-export-dynamic -lm -I. -ldl
 * freebsd:
 *   gcc -o repl repl.c s7.o -Wl,-export-dynamic -lm -I.
 * osx:
 *   gcc -o repl repl.c s7.o -lm -I.
 * openbsd:
 *   gcc -o repl repl.c s7.o -I. -ftrampolines -Wl,-export-dynamic -lm
 */
</pre>
</div>


<p>Since this reads stdin and writes stdout, it can be run as a Scheme subjob of emacs.
One (inconvenient) way to do this is to set the emacs variable scheme-program-name to
the name of the exectuable created above ("doc7"), then call the emacs function run-scheme:
M-x eval-expression in emacs, followed by (setq scheme-program-name "doc7"), then
M-x run-scheme, and you're talking to s7 in emacs.  Of course, this connection can be
customized indefinitely.  See, for example, inf-snd.el in the Snd package.
</p>

<p>Here are the not-always-built-in indentations I use in emacs:
</p>
<pre class="indented">
(put 'with-let 'scheme-indent-function 1)
(put 'with-baffle 'scheme-indent-function 0)
(put 'with-sound 'scheme-indent-function 1)
(put 'catch 'scheme-indent-function 1)
(put 'lambda* 'scheme-indent-function 1)
(put 'when 'scheme-indent-function 1)
(put 'let-temporarily 'scheme-indent-function 1)
(put 'let*-temporarily 'scheme-indent-function 1)
(put 'call-with-input-string 'scheme-indent-function 1)
(put 'unless 'scheme-indent-function 1)
(put 'letrec* 'scheme-indent-function 1)
(put 'sublet 'scheme-indent-function 1)
(put 'varlet 'scheme-indent-function 1)
</pre>

<p>To read stdin while working in a GUI-based program is trickier.  In glib/gtk, you can use
something like this:
</p>

<blockquote>
<div class="indented">
<pre>
static gboolean read_stdin(GIOChannel *source, GIOCondition condition, gpointer data)
{
  /* here read from g_io_channel_unix_get_fd(source) and call s7_eval_string */
  return(true);
}

/* ... during initialization ... */

GIOChannel *channel;
channel = g_io_channel_unix_new(STDIN_FILENO);  /* watch stdin */
stdin_id = g_io_add_watch_full(channel,         /* and call read_stdin above if input is noticed */
			       G_PRIORITY_DEFAULT, 
			       (GIOCondition)(G_IO_IN | G_IO_HUP | G_IO_ERR), 
			       <em class=red>read_stdin</em>, NULL, NULL);
g_io_channel_unref(channel);
</pre></div>
</blockquote>

<details>
<summary class="indented">repl with libtecla</summary>
<p>Here's a version that uses libtecla for the line editor:
</p>

<blockquote>
<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;libtecla.h&gt;
#include "s7.h"

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char *buffer;
  char response[1024];
  GetLine *gl;            /* The tecla line editor */

  gl = new_GetLine(500, 5000);
  s7 = s7_init();  

  while (1) 
    {
      buffer = gl_get_line(gl, "&gt; ", NULL, 0);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	  fprintf(stdout, "\n");
	}
    }
  gl = del_GetLine(gl);
}

/* 
 *   gcc -c s7.c -I. -O2 -g3
 *   gcc -o ex1 ex1.c s7.o -lm -I. -ltecla -ldl
 */
</pre></div>
</blockquote>
</details>

<p>A repl (based on repl.scm) is built into s7.  Include the compiler flag -DWITH_MAIN:
</p>

<pre class="indented">
in Linux: gcc s7.c -o repl -DWITH_MAIN -I. -O2 -g -ldl -lm -Wl,-export-dynamic
in *BSD:  gcc s7.c -o repl -DWITH_MAIN -I. -O2 -g -lm -Wl,-export-dynamic
in OSX:   gcc s7.c -o repl -DWITH_MAIN -I. -O2 -g -lm
</pre>


<details>
<summary class="indented">repl in C++</summary>
<p>If you prefer C++, here's a C++ version of the listener, extracted from Rick Taube's
Common Music package:
</p>

<blockquote>
<div class="indented">
<pre>
#include &lt;iostream&gt;
#include "s7.h"

static s7_pointer main_quit(s7_scheme *sc, s7_pointer args);
static bool is_balanced(std::string str);
static bool is_not_white(std::string str);

int main(int argc, const char* argv[])
{
  s7_scheme* s7 = s7_init();
  s7_pointer val;
  std::string str;

  try 
    {
      while (std::cin)
	{
	  std::cout &lt;&lt; "\ns7&gt; ";
	  str = "";
	  while (true)
	    {
	      std::string lin;
	      std::getline(std::cin, lin);
	      str = str + lin + "\n";
	      if (is_balanced(str))
		break;
	    }
	  if (is_not_white(str))
	    {
	      val = s7_eval_c_string(s7, str.c_str());
	      std::cout &lt;&lt; s7_object_to_c_string(s7, val);
	    }
	}
    }
  catch(...)
    {
    }
  std::cout &lt;&lt; "Bye!\n";
  return 0;
}

static s7_pointer main_quit(s7_scheme *sc, s7_pointer args)
{
  throw 0;
  return(s7_nil(sc));
}

static bool is_balanced(std::string str)
{
  int parens = 0;
  int quotes = 0;
  unsigned i = 0;
  while (i &lt; str.size())
    {
      if (str[i] == ';')
	{
	  for (i = i + 1; i &lt; str.size(); i++)
	    {
	      if (str[i] == '\n')
		break;
	    }
	}
      else if (str[i] == '"')
	{
	  if (i == 0 || str[i - 1] != '\\')
	    {
	      quotes = 1;
	      for (i = i + 1; i &lt; str.size(); i++)
		{
		  if (str[i] == '"' &amp;&amp; str[i - 1] != '\\')
		    {
		      quotes = 0;
		      break;
		    }
		}
	      if (quotes)
		return false;
	    }
	}
      else if (str[i] == '(')
	parens++;
      else if (str[i] == ')')
	parens--;
      i++;
    }
  return (parens == 0) &amp;&amp; (quotes == 0);
}

static bool is_not_white(std::string str)
{
  for (unsigned i = 0; (i &lt; str.size() &amp;&amp; str[i] != ';'); i++)
    if (str[i] != ' ' && str[i] != '\n' &amp;&amp; str[i] != '\t')
      return true;
  return false;
}

/* g++ -I. -c repl.cpp
 * g++ -o repl repl.o s7.o -ldl
 */
</pre></div>
</blockquote>
</details>


<p id="beginhook">
Common Lisp has something called "evalhook" that makes it possible
to insert your own function into the eval loop.  In s7, we have a "begin_hook" which sits at the opening of many begin blocks
(implicit or explicit).  begin_hook is a (C) function; 
if it sets its bool argument to true, 
s7 interrupts the current evaluation. 
Here is a version of the REPL in which begin_hook watches for C-g to interrupt
some long computation:
</p>

<blockquote>
<div class="indented">
<pre>
/* terminal-based REPL, 
 *    an expansion of the <a href="#repl">read-eval-print loop</a> program above.
 * type C-g to interrupt an evaluation.
 */
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;termios.h&gt;
#include &lt;signal.h&gt;

#include "s7.h"

static struct termios save_buf, buf;

static void sigcatch(int n)
{
  /* put things back the way they were */
  tcsetattr(fileno(stdin), TCSAFLUSH, &amp;save_buf);
  exit(0);
}

static char buffer[512];
static int type_ahead_point = 0;

static void <em class=red>watch_for_c_g</em>(s7_scheme *sc, bool *all_done)
{
  char c;
  /* watch for C-g without blocking, save other chars as type-ahead */
  tcsetattr(fileno(stdin), TCSAFLUSH, &amp;buf);
  if (read(fileno(stdin), &amp;c, 1) == 1)
    {
      if (c == 7) /* C-g */
	{
	  *all_done = true;
	  type_ahead_point = 0;
	}
      else buffer[type_ahead_point++] = c;
    }
  tcsetattr(fileno(stdin), TCSAFLUSH, &amp;save_buf);
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  bool use_begin_hook;

  use_begin_hook = (tcgetattr(fileno(stdin), &amp;save_buf) &gt;= 0);
  if (use_begin_hook)
    {
      buf = save_buf;
      buf.c_lflag &amp;= ~ICANON;
      buf.c_cc[VMIN] = 0;
      buf.c_cc[VTIME] = 0;

      signal(SIGINT, sigcatch);
      signal(SIGQUIT, sigcatch);
      signal(SIGTERM, sigcatch);
    }
  s7 = s7_init();  

  if (argc == 2)
    {
      fprintf(stderr, "load %s\n", argv[1]);
      s7_load(s7, argv[1]);
    }
  else
    {
      char response[1024];
      while (1) 
	{
	  fprintf(stdout, "\n&gt; ");
	  fgets((char *)(buffer + type_ahead_point), 512 - type_ahead_point, stdin);
	  type_ahead_point = 0;

	  if ((buffer[0] != '\n') || 
	      (strlen(buffer) &gt; 1))
	    {                            
	      sprintf(response, "(write %s)", buffer);

	      if (use_begin_hook)
		<em class=red>s7_set_begin_hook</em>(s7, watch_for_c_g);
	      s7_eval_c_string(s7, response);
	      if (use_begin_hook)
		<em class=red>s7_set_begin_hook</em>(s7, NULL);
	    }
	}
    }
  if (use_begin_hook)
    tcsetattr(fileno(stdin), TCSAFLUSH, &amp;save_buf);
}
</pre></div>
</blockquote>


<div class="header" id="defun"><h4>Define a function with arguments and a returned value, and a variable</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer add1(s7_scheme *sc, s7_pointer args)
{
  /* all added functions have this form, args is a list, 
   *    s7_car(args) is the first arg, etc 
   */
  if (<em class=red>s7_is_integer</em>(s7_car(args)))
    return(<em class=red>s7_make_integer</em>(sc, 1 + <em class=red>s7_integer</em>(s7_car(args))));
  return(s7_wrong_type_arg_error(sc, "add1", 1, s7_car(args), "an integer"));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  
  s7_define_function(s7, "add1", add1, 1, 0, false, "(add1 int) adds 1 to int");
                                      /* add the function "add1" to the interpreter.
                                       *   1, 0, false -&gt; one required arg,
				       *                  no optional args,
				       *                  no "rest" arg
				       */
 <em class=red>s7_define_variable</em>(s7, "my-pi", <em class=red>s7_make_real</em>(s7, 3.14159265));

  while (1)                           /* fire up a "repl" */
    {
      fprintf(stdout, "\n&gt; ");        /* prompt for input */
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
	}
    }
}

/*    doc7
 *    &gt; my-pi
 *    <em class="gray">3.14159265</em>
 *    &gt; (+ 1 (add1 1))
 *    <em class="gray">3</em>
 *    &gt; (exit)
 */
</pre></div>


<div class="header" id="defvar"><h4>Call a Scheme-defined function from C, and get/set Scheme variable values in C</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

int main(int argc, char **argv)
{
  s7_scheme *s7;
  s7 = s7_init();

  s7_define_variable(s7, "an-integer", s7_make_integer(s7, 1));
  s7_eval_c_string(s7, "(define (add1 a) (+ a 1))");
  
  fprintf(stderr, "an-integer: %lld\n", 
	  s7_integer(<em class=red>s7_name_to_value</em>(s7, "an-integer")));

  <em class=red>s7_symbol_set_value</em>(s7, <em class=red>s7_make_symbol</em>(s7, "an-integer"), s7_make_integer(s7, 32));

  fprintf(stderr, "now an-integer: %lld\n", 
	  s7_integer(<em class=red>s7_name_to_value</em>(s7, "an-integer")));

  fprintf(stderr, "(add1 2): %lld\n", 
	  s7_integer(<em class=red>s7_call</em>(s7, 
			     s7_name_to_value(s7, "add1"), 
			     s7_cons(s7, s7_make_integer(s7, 2), s7_nil(s7)))));
}

/*
 *    doc7
 *    an-integer: 1
 *    now an-integer: 32
 *    (add1 2): 3
 */
</pre></div>


<div class="header" id="juce"><h4>C++ and Juce, from Rick Taube</h4></div>


<div class="indented">
<pre>
int main(int argc, const char* argv[]) 
{ 
  initialiseJuce_NonGUI(); 

  s7_scheme *s7 = s7_init(); 
  if (!s7) 
    { 
      std::cout &lt;&lt;  "Can't start S7!\n"; 
      return -1; 
    } 

  s7_pointer val; 
  std::string str; 
  while (true) 
    { 
      std::cout &lt;&lt; "\ns7&gt; "; 
      std::getline(std::cin, str); 
      val = s7_eval_c_string(s7, str.c_str()); 
      std::cout &lt;&lt; s7_object_to_c_string(s7, val); 
    } 

  free(s7); 
  std::cout &lt;&lt; "Bye!\n"; 
  return 0; 
} 
</pre></div>


<div class="header" id="sndlib"><h4>Load sndlib into an s7 repl</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;unistd.h&gt;

/* assume we've configured and built sndlib, so it has created a mus-config.h file.
 * also assume we've built s7 with WITH_SYSTEM_EXTRAS set, so we have file-exists? and delete-file
 */

#include "mus-config.h"
#include "s7.h"
#include "xen.h"
#include "clm.h"
#include "clm2xen.h"

/* we need to redirect clm's mus_error calls to s7_error */

static void mus_error_to_s7(int type, char *msg)
{
  s7_error(s7,                               /* s7 is declared in xen.h, defined in xen.c */
	   s7_make_symbol(s7, "mus-error"),
	   s7_cons(s7, s7_make_string(s7, msg), s7_nil(s7)));
}

int main(int argc, char **argv)
{
  char buffer[512];
  char response[1024];

  s7 = s7_init();                     /* initialize the interpreter */
  s7_xen_initialize(s7);              /* initialize the xen stuff (hooks and the xen s7 FFI used by sndlib) */
  Init_sndlib();                      /* initialize sndlib with all the functions linked into s7 */  

  mus_error_set_handler(mus_error_to_s7); /* catch low-level errors and pass them to s7-error */

  while (1)                           /* fire up a "repl" */
    {
      fprintf(stdout, "\n&gt; ");        /* prompt for input */
      fgets(buffer, 512, stdin);

      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
	}
    }
}

/* gcc -o doc7 doc7.c -lm -I. /usr/local/lib/libsndlib.a -lasound -ldl
 *
 *   (load "sndlib-ws.scm")
 *   (with-sound () (outa 10 .1))
 *   (load "v.scm")
 *   (with-sound () (fm-violin 0 .1 440 .1))
 *
 * you might also need -lgsl -lgslcblas -lfftw3
 */
</pre>
</div>

<p>If you built libsndlib.so, it is possible to use it directly in the s7 repl:
</p>
<pre>
repl          ; this is a bare s7 running repl.scm via -DWITH_MAIN=1
loading libc_s7.so
&gt; (load "/home/bil/test/sndlib/libsndlib.so" (inlet 'init_func 's7_init_sndlib))
#t            ; s7_init_sndlib ties all the sndlib functions and variables into s7
&gt; (load "sndlib-ws.scm")
tmpnam
&gt; (set! *clm-player* (lambda (file) (system (format #f "sndplay ~A" file))))
&gt; (load "v.scm")
fm-violin
&gt; (with-sound (:play #t) (fm-violin 0 1 440 .1))
"test.snd"
</pre>

<p>You can use autoload to load libsndlib when needed:
</p>

<pre class="indented">
(define (find-library name)
  (if (or (file-exists? name)
	  (char=? (name 0) #\/))
      name
      (call-with-exit
       (lambda (return)
	 (for-each
	  (lambda (path)
	    (let ((new-name (string-append path "/" name)))
	      (if (file-exists? new-name)
		  (return new-name))))
	  *load-path*)
	 (let ((libs (getenv "LD_LIBRARY_PATH")) ; colon separated directory names
	       (start 0))
	   (do ((colon (char-position #\: libs) (char-position #\: libs start)))
	       ((or (not colon)
		    (let ((new-name (string-append (substring libs start colon) "/" name)))
		      (and (file-exists? new-name)
			   (return new-name)))))
	     (set! start (+ colon 1))))
	 name))))

(<em class=red>autoload</em> 'clm 
  (lambda (e)
    (load (find-library "libsndlib.so") (inlet '(init_func . s7_init_sndlib)))
    (set! *features* (cons 'clm *features*))
    (with-let (rootlet) (define clm #t))
    (load "sndlib-ws.scm")
    (set! *clm-player* (lambda (file) (system (format #f "sndplay ~A" file))))))
</pre>

<p>and use the repl's vt100 stuff to (for example) post the current begin time
as a note list computes:
</p>

<pre class="indented">
(define (clm-notehook . args)
  ;; assume second arg is begin time (first is instrument name)
  (when (and (pair? args) 
	     (pair? (cdr args)) 
	     (number? (cadr args)))
    (with-let (sublet (*repl* 'repl-let) :begin-time (cadr args))
      (let ((coords (cursor-coords))
	    (col (floor (/ last-col 2))))
	(let ((str (number-&gt;string begin-time)))
	  (format *stderr* "~C[~D;~DH" #\escape prompt-row col)
	  (format *stderr* "~C[K~A"  #\escape (if (&gt; (length str) col) (substring str 0 (- col 1)) str)))
	(format *stderr* "~C[~D;~DH"   #\escape (cdr coords) (car coords))))))

(set! *clm-notehook* clm-notehook)
</pre>


<div class="header" id="pwstype"><h4>Add a new Scheme type and a procedure with a setter</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

/* define *listener-prompt* in scheme, add two accessors for C get/set */

static const char *listener_prompt(s7_scheme *sc)
{
  return(s7_string(s7_name_to_value(sc, "*listener-prompt*")));
}

static void set_listener_prompt(s7_scheme *sc, const char *new_prompt)
{
  s7_symbol_set_value(sc, s7_make_symbol(sc, "*listener-prompt*"), s7_make_string(sc, new_prompt));
}

/* now add a new type, a struct named "dax" with two fields, a real "x" and a list "data" */
/*   since the data field is an s7 object, we'll need to mark it to protect it from the GC */

typedef struct {
  s7_double x;
  s7_pointer data;
} dax;

static char *print_dax(s7_scheme *sc, void *val)
{
  char *data_str, *str;
  int data_str_len;
  dax *o = (dax *)val;
  data_str = s7_object_to_c_string(sc, o-&gt;data);
  data_str_len = strlen(data_str);
  str = (char *)calloc(data_str_len + 32, sizeof(char));
  snprintf(str, data_str_len + 32, "#&lt;dax %.3f %s&gt;", o-&gt;x, data_str);
  free(data_str);
  return(str);
}

static void free_dax(void *val)
{
  if (val) free(val);
}

static bool equal_dax(void *val1, void *val2)
{
  return(val1 == val2);
}

static void mark_dax(void *val)
{
  dax *o = (dax *)val;
  if (o) s7_mark_object(o-&gt;data);
}

static int dax_type_tag = 0;

static s7_pointer make_dax(s7_scheme *sc, s7_pointer args)
{
  dax *o;
  o = (dax *)malloc(sizeof(dax));
  o-&gt;x = s7_real(s7_car(args));
  if (s7_cdr(args) != s7_nil(sc))
    o-&gt;data = s7_cadr(args);
  else o-&gt;data = s7_nil(sc);
  return(<em class=red>s7_make_object</em>(sc, dax_type_tag, (void *)o));
}

static s7_pointer is_dax(s7_scheme *sc, s7_pointer args)
{
  return(s7_make_boolean(sc, 
			 <em class=red>s7_is_object</em>(s7_car(args)) &amp;&amp;
			 <em class=red>s7_object_type</em>(s7_car(args)) == dax_type_tag));
}

static s7_pointer dax_x(s7_scheme *sc, s7_pointer args)
{
  dax *o;
  o = (dax *)<em class=red>s7_object_value</em>(s7_car(args));
  return(s7_make_real(sc, o-&gt;x));
}

static s7_pointer set_dax_x(s7_scheme *sc, s7_pointer args)
{
  dax *o;
  o = (dax *)s7_object_value(s7_car(args));
  o-&gt;x = s7_real(s7_cadr(args));
  return(s7_cadr(args));
}

static s7_pointer dax_data(s7_scheme *sc, s7_pointer args)
{
  dax *o;
  o = (dax *)s7_object_value(s7_car(args));
  return(o-&gt;data);
}

static s7_pointer set_dax_data(s7_scheme *sc, s7_pointer args)
{
  dax *o;
  o = (dax *)s7_object_value(s7_car(args));
  o-&gt;data = s7_cadr(args);
  return(o-&gt;data);
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  
  s7_define_variable(s7, "*listener-prompt*", s7_make_string(s7, "&gt;"));

  dax_type_tag = <em class=red>s7_new_type</em>("dax", print_dax, free_dax, equal_dax, mark_dax, NULL, NULL);
  s7_define_function(s7, "make-dax", make_dax, 2, 0, false, "(make-dax x data) makes a new dax");
  s7_define_function(s7, "dax?", is_dax, 1, 0, false, "(dax? anything) returns #t if its argument is a dax object");

  s7_define_variable(s7, "dax-x", 
                     <em class=red>s7_dilambda</em>(s7, "dax-x", dax_x, 1, 0, set_dax_x, 2, 0, "dax x field"));

  s7_define_variable(s7, "dax-data", 
                     <em class=red>s7_dilambda</em>(s7, "dax-data", dax_data, 1, 0, set_dax_data, 2, 0, "dax data field"));

  while (1)
    {
      fprintf(stdout, "\n%s ", listener_prompt(s7));
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
	}
    }
}

/*
 *    &gt; *listener-prompt*
 *    <em class="gray">"&gt;"</em>
 *    &gt; (set! *listener-prompt* ":")
 *    <em class="gray">":"</em>
 *    : (define obj (make-dax 1.0 (list 1 2 3)))
 *    <em class="gray">obj</em>
 *    : obj
 *    <em class="gray">#&lt;dax 1.000 (1 2 3)&gt;</em>
 *    : (dax-x obj)
 *    <em class="gray">1.0</em>
 *    : (dax-data obj)
 *    <em class="gray">(1 2 3)</em>
 *    : (set! (dax-x obj) 123.0)
 *    <em class="gray">123.0</em>
 *    : obj
 *    <em class="gray">#&lt;dax 123.000 (1 2 3)&gt;</em>
 *    : (dax? obj)
 *    <em class="gray">#t</em>
 *    : (exit)
 */
</pre></div>


<div class="header" id="functionportexample"><h4>Redirect output (and input) to a C procedure</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static void my_print(s7_scheme *sc, unsigned char c, s7_pointer port)
{
  fprintf(stderr, "[%c] ", c);
}

static s7_pointer my_read(s7_scheme *sc, s7_read_t peek, s7_pointer port)
{
  return(<em class=red>s7_make_character</em>(sc, fgetc(stdin)));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();  

  <em class=red>s7_set_current_output_port</em>(s7, <em class=red>s7_open_output_function</em>(s7, my_print));
  s7_define_variable(s7, "io-port", <em class=red>s7_open_input_function</em>(s7, my_read));

  while (1) 
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* 
 *    &gt; (+ 1 2)
 *    <em class="gray">[3]</em>
 *    &gt; (display "hiho")
 *    <em class="gray">[h] [i] [h] [o] [#] [&lt;] [u] [n] [s] [p] [e] [c] [i] [f] [i] [e] [d] [&gt;] </em>
 *    &gt; (define (add1 x) (+ 1 x))
 *    <em class="gray">[a] [d] [d] [1] </em>
 *    &gt; (add1 123)
 *    <em class="gray">[1] [2] [4] </em>
 *    &gt; (read-char io-port)
 *    a                             ; here I typed "a" in the shell
 *    <em class="gray">[#] [\] [a] </em>
 */
</pre></div>


<div class="header" id="extendop"><h4>Extend a built-in operator ("+" in this case)</h4></div>

<p>There are several ways to do this.  In the first example, we save the original function,
and replace it with ours, calling the original whenever possible:
</p>

<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer old_add;           /* the original "+" function for non-string cases */
static s7_pointer old_string_append; /* same, for "string-append" */

static s7_pointer our_add(s7_scheme *sc, s7_pointer args)
{
  /* this will replace the built-in "+" operator, extending it to include strings:
   *   (+ "hi" "ho") -&gt; "hiho" and  (+ 3 4) -&gt; 7
   */
  if ((s7_is_pair(args)) &amp;&amp;
      (s7_is_string(s7_car(args))))
    return(<em class=red>s7_apply_function</em>(sc, old_string_append, args));
  return(s7_apply_function(sc, old_add, args));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];
  s7 = s7_init();

  /* get built-in + and string-append */
  old_add = s7_name_to_value(s7, "+");      
  old_string_append = s7_name_to_value(s7, "string-append");

  /* redefine "+" */
  s7_define_function(s7, "+", our_add, 0, 0, true, "(+ ...) adds or appends its arguments");

  while (1)
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/*    &gt; (+ 1 2)
 *    <em class="gray">3</em>
 *    &gt; (+ "hi" "ho")
 *    <em class="gray">"hiho"</em>
 */
</pre></div>

<p>In the next example, we use the method (inlet) machinery:
</p>

<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;math.h&gt;

#include "s7.h"

static s7_pointer our_abs(s7_scheme *sc, s7_pointer args)
{
  s7_pointer x;
  x = s7_car(args);
  if (!s7_is_number(x))
    {
      s7_pointer method;
      method = <em class=red>s7_method</em>(sc, x, s7_make_symbol(sc, "abs"));
      if (method == s7_undefined(sc))                       /* no method found, so raise an error */
	s7_wrong_type_arg_error(sc, "abs", 1, x, "a real"); 
      return(s7_apply_function(sc, method, args));          /*   else apply the method to the args */
    }
  return(s7_make_real(sc, (s7_double)fabs(s7_number_to_real(sc, x))));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  s7_define_function(s7, "our-abs", our_abs, 1, 0, false, "abs replacement");

  while (1)
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/*    &gt; (our-abs -1)
 *    <em class="gray">1.0</em>
 *    &gt; (our-abs (openlet (inlet 'value -3.0 'abs (lambda (x) (abs (x 'value))))))
 *    <em class="gray">3.0</em>
 */

</pre>
</div>


<div class="header" id="definestar1"><h4>C-side define* (s7_define_function_star)</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer plus(s7_scheme *sc, s7_pointer args)
{
  /* (define* (plus (red 32) blue) (+ (* 2 red) blue)) */
  return(s7_make_integer(sc, 2 * s7_integer(s7_car(args)) + s7_integer(s7_cadr(args))));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  <em class=red>s7_define_function_star</em>(s7, "plus", plus, "(red 32) blue", "an example of define* from C");

  while (1)
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* 
 *    &gt; (plus 2 3)
 *    <em class="gray">7</em>
 *    &gt; (plus :blue 3)
 *    <em class="gray">67</em>
 *    &gt; (plus :blue 1 :red 4)
 *    <em class="gray">9</em>
 *    &gt; (plus 2 :blue 3)
 *    <em class="gray">7</em>
 *    &gt; (plus :blue 3 :red 1)
 *    <em class="gray">5</em>
 */
</pre></div>


<div class="header" id="definemacro1"><h4>C-side define-macro (s7_define_macro)</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer plus(s7_scheme *sc, s7_pointer args)
{
  /* (define-macro (plus a b) `(+ ,a ,b)) */
  s7_pointer a, b;
  a = s7_car(args);
  b = s7_cadr(args);
  return(s7_list(sc, 3, s7_make_symbol(sc, "+"),  a, b));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  <em class=red>s7_define_macro</em>(s7, "plus", plus, 2, 0, false, "plus adds its two arguments");

  while (1)
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* 
 *    &gt; (plus 2 3)
 *    <em class="gray">5</em>
 */
</pre></div>


<div class="header" id="definegeneric"><h4>define a generic function in C</h4></div>

<p>In scheme, a function becomes generic simply by <code>(apply ((car args) 'func) args)</code>.
To accomplish the same thing in C, we use s7_method and s7_apply_function:
</p>

<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer plus(s7_scheme *sc, s7_pointer args)
{
  #define plus_help "(plus obj ...) applies obj's plus method to obj and any trailing arguments."
  s7_pointer obj, method;
  obj = s7_car(args);
  method = <em class=red>s7_method</em>(sc, obj, s7_make_symbol(sc, "plus"));
  if (s7_is_procedure(method))
    return(<em class=red>s7_apply_function</em>(sc, method, args));
  return(s7_f(sc));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  s7 = s7_init();
  s7_define_function(s7, "plus", plus, 1, 0, true, plus_help);
  while (1)
    {
      char buffer[512];
      char response[1024];
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* gcc -c s7.c -I.
 * gcc -o ex15 ex15.c s7.o -I. -lm -ldl
 *
 *     &gt; (plus 1 2)
 *     <em class="gray">#f</em>
 *     &gt; (define obj (openlet (inlet 'plus (lambda args (apply + 1 (cdr args))))))
 *     <em class="gray">obj</em>
 *     &gt; (plus obj 2 3)
 *     <em class="gray">6</em>
 */
</pre>
</div>


<div class="header" id="signal"><h4>Signal handling and continuations</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;signal.h&gt;

#include "s7.h"

static s7_scheme *s7;
struct sigaction new_act, old_act;  
  
static void handle_sigint(int ignored)  
{  
  fprintf(stderr, "interrupted!\n");
  s7_symbol_set_value(s7, s7_make_symbol(s7, "*interrupt*"), <em class=red>s7_make_continuation</em>(s7)); /* save where we were interrupted */
  sigaction(SIGINT, &amp;new_act, NULL);  
  s7_quit(s7);                             /* get out of the eval loop if possible */
}  

static s7_pointer our_sleep(s7_scheme *sc, s7_pointer args)
{
  /* slow down our infinite loop for demo purposes */
  sleep(1);
  return(s7_f(sc));
}

int main(int argc, char **argv)
{
  char buffer[512];
  char response[1024];

  s7 = s7_init();
  s7_define_function(s7, "sleep", our_sleep, 0, 0, false, "(sleep) sleeps");
  s7_define_variable(s7, "*interrupt*", s7_f(s7)); 
  /* Scheme variable *interrupt* holds the continuation at the point of the interrupt */

  sigaction(SIGINT, NULL, &amp;old_act);
  if (old_act.sa_handler != SIG_IGN)
    {
      memset(&amp;new_act, 0, sizeof(new_act));  
      new_act.sa_handler = &amp;handle_sigint;  
      sigaction(SIGINT, &amp;new_act, NULL);  
    }

  while (1)
    {
      fprintf(stderr, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/*
 *    &gt; (do ((i 0 (+ i 1))) ((= i -1)) (format () "~D " i) (sleep))
 *      ;;; now type C-C to break out of this loop
 *    0 1 2 ^Cinterrupted!
 *      ;;; call the continuation to continue from where we were interrupted
 *    &gt; (*interrupt*)
 *    3 4 5 ^Cinterrupted!
 *    &gt; *interrupt*
 *    #&lt;continuation&gt;
 *    &gt; (+ 1 2)
 *    3
 */
</pre></div>


<div class="header" id="vector"><h4>Multidimensional vector element access</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;stdarg.h&gt;

#include "s7.h"

static s7_pointer multivector_ref(s7_scheme *sc, s7_pointer vector, int indices, ...)
{
  /* multivector_ref returns an element of a multidimensional vector */
  int ndims;
  ndims = <em class=red>s7_vector_rank</em>(vector);

  if (ndims == indices)
    {
      va_list ap;
      s7_int index = 0;
      va_start(ap, indices);

      if (ndims == 1)
	{
	  index = va_arg(ap, s7_int);
	  va_end(ap);
	  return(s7_vector_ref(sc, vector, index));
	}
      else
	{
	  int i;
	  s7_pointer *elements;
	  s7_int *offsets, *dimensions;

	  elements = <em class=red>s7_vector_elements</em>(vector);
	  dimensions = <em class=red>s7_vector_dimensions</em>(vector);
	  offsets = <em class=red>s7_vector_offsets</em>(vector);

	  for (i = 0; i &lt; indices; i++)
	    {
	      int ind;
	      ind = va_arg(ap, int);
	      if ((ind &lt; 0) ||
		  (ind &gt;= dimensions[i]))
		{
		  va_end(ap);
		  return(s7_out_of_range_error(sc, 
                                               "multivector_ref", i, 
                                               s7_make_integer(sc, ind), 
                                               "index should be between 0 and the dimension size"));
		}
	      index += (ind * offsets[i]);
	    }
	  va_end(ap);
	  return(elements[index]);
	}
    }
  return(s7_wrong_number_of_args_error(sc, 
                                       "multivector_ref: wrong number of indices: ~A", 
                                       s7_make_integer(sc, indices)));
}

int main(int argc, char **argv)
{
  char buffer[512];
  char response[1024];
  s7_scheme *s7;

  s7 = s7_init(); 
  s7_eval_c_string(s7, "(define vect (make-vector '(2 3 4) 0))");
  s7_eval_c_string(s7, "(set! (vect 1 1 1) 32)");

  fprintf(stdout, "vect[0,0,0]: %s, vect[1,1,1]: %s\n",
	  s7_object_to_c_string(s7, <em class=red>multivector_ref</em>(s7, s7_name_to_value(s7, "vect"), 3, 0, 0, 0)),
	  s7_object_to_c_string(s7, <em class=red>multivector_ref</em>(s7, s7_name_to_value(s7, "vect"), 3, 1, 1, 1)));
}

/* vect[0,0,0]: 0, vect[1,1,1]: 32
 */
</pre>
</div>

<p>Much later... I decided to add s7_vector_ref_n and s7_vector_set_n to s7.
</p>


<div class="header" id="notify"><h4>Notification from Scheme that a given Scheme variable has been set</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer scheme_set_notification(s7_scheme *sc, s7_pointer args)
{
  /* this function is called when the Scheme variable is set! */
  fprintf(stderr, "%s set to %s\n",
	  s7_object_to_c_string(sc, s7_car(args)),
	  s7_object_to_c_string(sc, s7_cadr(args)));
  return(s7_cadr(args));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  s7 = s7_init();  

  s7_define_function(s7, "notify-C", scheme_set_notification, 2, 0, false, "called if notified-var is set!");
  s7_define_variable(s7, "notified-var", s7_make_integer(s7, 0));
  <em class=red>s7_symbol_set_access</em>(s7, s7_make_symbol(s7, "notified-var"), s7_name_to_value(s7, "notify-C"));

  if (argc == 2)
    {
      fprintf(stderr, "load %s\n", argv[1]);
      s7_load(s7, argv[1]);
    }
  else
    {
      char buffer[512];
      char response[1024];
      while (1) 
	{
	  fprintf(stdout, "\n&gt; ");
	  fgets(buffer, 512, stdin);
	  
	  if ((buffer[0] != '\n') || 
	      (strlen(buffer) &gt; 1))
	    {                            
	      sprintf(response, "(write %s)", buffer);
	      s7_eval_c_string(s7, response);
	    }
	}
    }
}

/*    &gt; notified-var
 *    <em class="gray">0</em>
 *    &gt; (set! notified-var 32)
 *    <em class="gray">notified-var set to 32</em>
 *    <em class="gray">32</em>
 */
</pre></div>


<div class="header" id="namespace"><h4>Load C defined stuff into a separate namespace</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer func1(s7_scheme *sc, s7_pointer args)
{
  return(s7_make_integer(sc, s7_integer(s7_car(args)) + 1));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  s7_pointer new_env;

  s7 = s7_init();  

  /* "func1" and "var1" will be placed in an anonymous environment,
   *   accessible from Scheme via the global variable "lib-exports"
   */
  
  new_env = <em class=red>s7_inlet</em>(s7, s7_curlet(s7), s7_nil(s7));
  /* make a private environment for func1 and var1 below (this is our "namespace") */
  s7_gc_protect(s7, new_env);

  s7_define(s7, <em class=red>new_env</em>, 
	    s7_make_symbol(s7, "func1"),
	    <em class=red>s7_make_function</em>(s7, "func1", func1, 1, 0, false, "func1 adds 1 to its argument"));
  
  s7_define(s7, <em class=red>new_env</em>, s7_make_symbol(s7, "var1"), s7_make_integer(s7, 32));
  /* those two symbols are now defined in the new environment */

  /* add "lib-exports" to the global environment */
  s7_define_variable(s7, "lib-exports", <em class=red>s7_let_to_list</em>(s7, new_env));

  if (argc == 2)
    {
      fprintf(stderr, "load %s\n", argv[1]);
      s7_load(s7, argv[1]);
    }
  else
    {
      char buffer[512];
      char response[1024];
      while (1) 
	{
	  fprintf(stdout, "\n&gt; ");
	  fgets(buffer, 512, stdin);
	  
	  if ((buffer[0] != '\n') || 
	      (strlen(buffer) &gt; 1))
	    {                            
	      sprintf(response, "(write %s)", buffer);
	      s7_eval_c_string(s7, response);
	    }
	}
    }
}

/*     &gt; func1
 *     <em class="gray">;func1: unbound variable, line 1</em>
 *     &gt; lib-exports
 *     <em class="gray">((var1 . 32) (func1 . func1))</em>
 *     ;; so lib-exports has the C-defined names and values
 *     ;; we can use these directly:
 *
 *     &gt; (define lib-env (apply <em class=red>sublet</em> (curlet) lib-exports))
 *     <em class="gray">lib-env</em>
 *     &gt; (<em class=red>with-let</em> lib-env (func1 var1))
 *     <em class="gray">33</em>
 *
 *     ;; or rename them to prepend "lib:"
 *     &gt; (define lib-env (apply sublet 
                                (curlet) 
                                (map (lambda (binding) 
                                       (cons (string-&gt;symbol 
                                               (string-append "lib:" (symbol-&gt;string (car binding)))) 
                                             (cdr binding))) 
                                     lib-exports)))
 *     <em class="gray">lib-env</em>
 *     &gt; (with-let lib-env (lib:func1 lib:var1))
 *     <em class="gray">33</em>
 *
 *     ;;; now for convenience, place "func1" in the global environment under the name "func2"
 *     &gt; (define func2 (cdadr lib-exports)) 
 *     <em class="gray">func2</em>
 *     &gt; (func2 1)  
 *     <em class="gray">2</em>
 */
</pre></div>


<div class="header" id="Cerrors"><h4>Handle scheme errors in C</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer error_handler(s7_scheme *sc, s7_pointer args)
{
  fprintf(stdout, "error: %s\n", s7_string(s7_car(args)));
  return(s7_f(sc));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];
  bool with_error_hook = false;

  s7 = s7_init();  
  s7_define_function(s7, "error-handler", error_handler, 1, 0, false, "our error handler");

  if (with_error_hook)
    s7_eval_c_string(s7, "(set! (hook-functions *error-hook*)                    \n\
                            (list (lambda (hook)                                 \n\
                                    (error-handler                               \n\
                                      (apply format #f (hook 'data)))            \n\
                                    (set! (hook 'result) 'our-error))))");
  while (1) 
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
	  
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  s7_pointer old_port, result;
	  int gc_loc = -1;
	  const char *errmsg = NULL;

	  /* trap error messages */
	  old_port = s7_set_current_error_port(s7, s7_open_output_string(s7));
	  if (old_port != s7_nil(s7))
	    gc_loc = s7_gc_protect(s7, old_port);

	  /* evaluate the input string */
	  result = s7_eval_c_string(s7, buffer);

	  /* print out the value wrapped in "{}" so we can tell it from other IO paths */
	  fprintf(stdout, "{%s}", s7_object_to_c_string(s7, result));

	  /* look for error messages */
	  errmsg = s7_get_output_string(s7, s7_current_error_port(s7));

	  /* if we got something, wrap it in "[]" */
	  if ((errmsg) &amp;&amp; (*errmsg))
	    fprintf(stdout, "[%s]", errmsg); 

	  s7_close_output_port(s7, s7_current_error_port(s7));
	  s7_set_current_error_port(s7, old_port);
	  if (gc_loc != -1)
	    s7_gc_unprotect_at(s7, gc_loc);
	}
    }
}

/* 
 *   gcc -c s7.c -I. -g3
 *   gcc -o ex3 ex3.c s7.o -lm -I. -ldl
 *
 * if with_error_hook is false,
 *
 *   &gt; (+ 1 2)
 *   <em class="gray">{3}</em>
 *   &gt; (+ 1 #\c)
 *   <em class="gray">{wrong-type-arg}[</em>
 *   <em class="gray">;+ argument 2, #\c, is character but should be a number, line 1</em>
 *   ]
 *
 * so s7 by default prepends ";" to the error message, and appends "\n",
 *   sending that to current-error-port, and the error type ('wrong-type-arg here)
 *   is returned.
 *
 * if with_error_hook is true,
 *
 *   &gt; (+ 1 2)
 *   <em class="gray">{3}</em>
 *   &gt; (+ 1 #\c)
 *   <em class="gray">error: + argument 2, #\c, is character but should be a number</em>
 *   <em class="gray">{our-error}</em>
 *
 * so now the *error-hook* code handles both the error reporting and
 *   the value returned ('our-error in this case).
 */
</pre></div>


<div class="header" id="testhook"><h4>C and Scheme hooks</h4></div>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

static s7_pointer my_hook_function(s7_scheme *sc, s7_pointer args)
{
  fprintf(stderr, "a is %s\n", s7_object_to_c_string(sc, s7_symbol_local_value(sc, s7_make_symbol(sc, "a"), s7_car(args))));
  return(s7_car(args));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];
  s7_pointer test_hook;

  s7 = s7_init();  

  /* define test_hook in C, test-hook in Scheme, arguments are named a and b */
  test_hook = <em class=red>s7_eval_c_string</em>(s7, "(make-hook 'a 'b)");
  s7_define_constant(s7, "test-hook", test_hook); 

  /* add my_hook_function to the test_hook function list */
  <em class=red>s7_hook_set_functions</em>(s7, test_hook, 
			s7_cons(s7, 
				s7_make_function(s7, "my-hook-function", my_hook_function, 1, 0, false, "my hook-function"), 
				s7_hook_functions(s7, test_hook)));
  while (1) 
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
	  
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* 
 *    &gt; test-hook
 *    <em class="gray">#&lt;lambda (hook)&gt;</em>
 *    &gt; (hook-functions test-hook)
 *    <em class="gray">(my-hook-function)</em>
 *    &gt; (test-hook 1 2)
 *    <em class="gray">a is 1</em>
 *    <em class="gray">#&lt;unspecified&gt;</em>
 */
</pre></div>


<div class="header" id="dload"><h4>Load a shared library</h4></div>

<p>We can use dlopen to load a shared library, and dlsym to initialize
that library in our main program.  The tricky part is to conjure up the right
compiler and loader flags.
First we define a module that defines a new s7 function, add-1 that we'll tie
into s7 explicitly, and another
function that we'll try to call by waving a wand.
</p>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"

double a_function(double an_arg);
double a_function(double an_arg)
{
  return(an_arg + 1.0);
}

static s7_pointer add_1(s7_scheme *sc, s7_pointer args) 
{
  return(s7_make_integer(sc, s7_integer(s7_car(args)) + 1)); 
}

void init_ex(s7_scheme *sc);
void init_ex(s7_scheme *sc)  /* this needs to be globally accessible (not "static") */
{
  /* tell s7 about add-1, but leave a_function hidden */
  s7_define_function(sc, "add-1", add_1, 1, 0, false, "(add-1 x) adds 1 to x");
}

</pre></div>


<p>And here is our main program:
</p>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include "s7.h"
#include &lt;dlfcn.h&gt;

static void *library = NULL;

static s7_pointer try(s7_scheme *sc, s7_pointer args)
{
  /* try tries to call an arbitrary function in the shared library */
  void *func;
  func = <em class=red>dlsym</em>(library, s7_string(s7_car(args)));
  if (func)
    {
      /* we'll assume double f(double) */
      typedef double (*dl_func)(double arg);
      return(s7_make_real(sc, ((dl_func)<em class=red>func</em>)(s7_real(s7_cadr(args)))));
    }
  return(s7_error(sc, s7_make_symbol(sc, "can't find function"), 
		  s7_list(sc, 2, s7_make_string(sc, "loader error: ~S"), 
			         s7_make_string(sc, dlerror()))));
}

static s7_pointer cload(s7_scheme *sc, s7_pointer args)
{
  /* cload loads a shared library */
  #define CLOAD_HELP "(cload so-file-name) loads the module"
  library = dlopen(s7_string(s7_car(args)), RTLD_LAZY);
  if (library)
    {
      /* call our init func to define add-1 in s7 */
      void *init_func;
      init_func = <em class=red>dlsym</em>(library, s7_string(s7_cadr(args)));
      if (init_func)
	{
	  typedef void *(*dl_func)(s7_scheme *sc);
	  ((dl_func)<em class=red>init_func</em>)(sc);  /* call the initialization function (init_ex above) */
	  return(s7_t(sc));
	}
    }
  return(s7_error(sc, s7_make_symbol(sc, "load-error"), 
		      s7_list(sc, 2, s7_make_string(sc, "loader error: ~S"), 
			             s7_make_string(sc, dlerror()))));
}

int main(int argc, char **argv)
{
  char buffer[512];
  char response[1024];
  s7_scheme *s7;

  s7 = s7_init();  

  s7_define_function(s7, "cload", cload, 2, 0, false, CLOAD_HELP);
  s7_define_function(s7, "try", try, 2, 0, false, 
                         "(try name num) tries to call name in the shared library with the argument num.");

  while (1) 
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
	  
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* Put the module in the file ex3a.c and the main program in ex3.c, then
 *
 * in Linux:
 *   gcc -c -fPIC ex3a.c
 *   gcc ex3a.o -shared -o ex3a.so
 *   gcc -c s7.c -I. -fPIC -shared
 *   gcc -o ex3 ex3.c s7.o -lm -ldl -I. -Wl,-export-dynamic
 *   # omit -ldl in freeBSD, openBSD might want -ftrampolines
 *
 * in Mac OSX:
 *   gcc -c ex3a.c
 *   gcc ex3a.o -o ex3a.so -dynamic -bundle -undefined suppress -flat_namespace
 *   gcc -c s7.c -I. -dynamic -bundle -undefined suppress -flat_namespace
 *   gcc -o ex3 ex3.c s7.o -lm -ldl -I.
 *
 * and run it:
 *   ex3
 *   &gt; (cload "/home/bil/snd-16/ex3a.so" "init_ex")
 *   <em class="gray">#t</em>
 *   &gt; (add-1 2)
 *   <em class="gray">3</em>
 *   &gt; (try "a_function" 2.5)
 *   <em class="gray">3.5</em>
 */
</pre></div>

<p>All of this is just boring boilerplate, so with a little support from s7,
we can write a script to do the entire linkage.  The s7 side is an extension
to "load" that loads a shared object file if its extension is "so", and
runs an initialization function whose name is defined in the load
environment (the optional second argument to load).  An example of the scheme side is cload.scm,
included in the s7 tarball.  It defines a function that can be
called:
</p>

<pre class="indented">
(c-define '(double j0 (double)) "m" "math.h")
</pre>

<p>This links the s7 function m:j0 to the math library
function j0.  See <a href="#cload">cload.scm</a> for more details.
</p>


<div class="header" id="gmpex"><h4>Bignums in C</h4></div>

<p>Bignum support depends on gmp, mpfr, and mpc.  In this example, we define "add-1" which adds
1 to any kind of number.  The s7_big_* functions return the underlying gmp/mpfr/mpc pointer,
so we have to copy that into a new number before adding.
</p>


<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

#include &lt;gmp.h&gt;
#include &lt;mpfr.h&gt;
#include &lt;mpc.h&gt;

#include "s7.h"

static s7_pointer big_add_1(s7_scheme *sc, s7_pointer args)
{
  /* add 1 to either a normal number or a bignum */
  s7_pointer x;
  x = s7_car(args);
  if (s7_is_bignum(x))
    {
      s7_pointer n;
      if (s7_is_integer(x))
	{
	  mpz_t *big_n;
	  n = s7_make_big_integer(sc, s7_big_integer(x)); /* copy x */
	  big_n = s7_big_integer(n);                      /* get mpz_t pointer of copy */
	  mpz_add_ui(*big_n, *big_n, 1);                  /* add 1 to that */
	  return(n);                                      /* return the new bignum */
	}
      if (s7_is_ratio(x))
	{
	  mpq_t *big_q;
	  mpz_t num, den;
	  n = s7_make_big_ratio(sc, s7_big_ratio(x));
	  big_q = s7_big_ratio(n);
	  mpz_init_set(num, mpq_numref(*big_q));
	  mpz_init_set(den, mpq_denref(*big_q));
	  mpz_add(num, num, den);
	  mpq_set_num(*big_q, num);
	  mpz_clear(num);
	  mpz_clear(den);
	  return(n);
	}
      if (s7_is_real(x))
	{
	  mpfr_t *big_x;
	  n = s7_make_big_real(sc, s7_big_real(x));
	  big_x = s7_big_real(n);
	  mpfr_add_ui(*big_x, *big_x, 1, GMP_RNDN);
	  return(n);
	}
      /* x must be big complex */
      {
	mpc_t *big_z;
	n = s7_make_big_complex(sc, s7_big_complex(x));
	big_z = s7_big_complex(n);
	mpc_add_ui(*big_z, *big_z, 1, MPC_RNDNN);
	return(n);
      }
    }
  else
    {
      if (s7_is_integer(x))
	return(s7_make_integer(sc, 1 + s7_integer(x)));
      if (s7_is_rational(x))
	return(s7_make_ratio(sc, s7_numerator(x) + s7_denominator(x), s7_denominator(x)));
      if (s7_is_real(x))
	return(s7_make_real(sc, 1.0 + s7_real(x)));
      if (s7_is_complex(x))
	return(s7_make_complex(sc, 1.0 + s7_real_part(x), s7_imag_part(x)));
    }
  return(s7_wrong_type_arg_error(sc, "add-1", 0, x, "a number"));
}

int main(int argc, char **argv)
{
  s7_scheme *s7;
  char buffer[512];
  char response[1024];

  s7 = s7_init();  
  s7_define_function(s7, "add-1", big_add_1, 1, 0, false, "(add-1 num) adds 1 to num");

  while (1) 
    {
      fprintf(stdout, "\n&gt; ");
      fgets(buffer, 512, stdin);
      if ((buffer[0] != '\n') || 
	  (strlen(buffer) &gt; 1))
	{                            
	  sprintf(response, "(write %s)", buffer);
	  s7_eval_c_string(s7, response);
	}
    }
}

/* 
 *   gcc -DWITH_GMP=1 -c s7.c -I. -O2 -g3
 *   gcc -DWITH_GMP=1 -o ex2 ex2.c s7.o -I. -O2 -lm -ldl -lgmp -lmpfr -lmpc
 *
 *   ex2
 *   &gt; (add-1 1)   
 *   <em class="gray">2</em>
 *   &gt; (add-1 2/3)
 *   <em class="gray">5/3</em>
 *   &gt; (add-1 1.4) 
 *   <em class="gray">2.4</em>
 *   &gt; (add-1 1.5+i)
 *   <em class="gray">2.5+1i</em>
 *   &gt; (add-1 (bignum "3"))
 *   <em class="gray">4</em>          ; this is the bignum 4
 *   &gt; (add-1 (bignum "3/4"))
 *   <em class="gray">7/4</em>
 *   &gt; (add-1 (bignum "1.4"))
 *   <em class="gray">2.399999999999999911182158029987476766109E0</em>
 *   &gt; (add-1 (bignum "1.5+i"))
 *   <em class="gray">2.500E0+1.000E0i</em>
 */
</pre></div>


<div class="header" id="glistener"><h4>glistener.c</h4></div>

<p>glistener.c is a gtk-based repl.  It is not specific to s7:
Snd uses it as its Forth and Ruby listener as well as for s7.  
Here's a short example:
</p>

<div class="indented">
<pre>
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;stdbool.h&gt;

#include &lt;gtk/gtk.h&gt;

#include "s7.h"
#include "glistener.h"

static s7_scheme *s7;

static gint quit_repl(GtkWidget *w, GdkEvent *event, gpointer context) {exit(0);}

static void evaluator(glistener *g, const char *text)
{
  /* this sends "text" to s7 for evaluation, then displays the result */
  int gc_loc;
  s7_pointer old_port, result;
  const char *errmsg = NULL;
  char *msg = NULL;
  
  old_port = s7_set_current_error_port(s7, s7_open_output_string(s7));
  gc_loc = s7_gc_protect(s7, old_port);
  
  result = s7_eval_c_string(s7, text);
  errmsg = s7_get_output_string(s7, s7_current_error_port(s7));
  if ((errmsg) && (*errmsg))
    {
      msg = (char *)calloc(strlen(errmsg) + 1, sizeof(char));
      strcpy(msg, errmsg);
    }
  
  s7_close_output_port(s7, s7_current_error_port(s7));
  s7_set_current_error_port(s7, old_port);
  s7_gc_unprotect_at(s7, gc_loc);
  
  glistener_append_text(g, "\n");
  if (msg)                      /* some error occurred during evaluation */
    glistener_append_text(g, msg);
  else 
    {                           /* evaluation produced an s7 object which we need to display */
      msg = s7_object_to_c_string(s7, result);
      glistener_append_text(g, msg);
    }
  if (msg) free(msg);
  glistener_append_prompt(g);  /* prompt for more input */
}

static void listener_init(glistener *g, GtkWidget *w)
{
  /* this is the glistener initialization function. "w" above is the new text-view widget,
   *   "g" is the new glistener pointer, passed to any function that wants to talk to this
   *   listener. 
   */
  unsigned char prompt[4] = {0xce, 0xbb, '&gt;', '\0'}; /* lambda as prompt */
  GtkTextBuffer *buffer;

  buffer = gtk_text_view_get_buffer(GTK_TEXT_VIEW(w));
  glistener_set_font(g, pango_font_description_from_string("Monospace 10"));

  /* our prompt will be a red lambda */
  glistener_set_prompt_tag(g, gtk_text_buffer_create_tag(buffer, "glistener_prompt_tag", 
							 "weight", PANGO_WEIGHT_BOLD, 
							 "foreground", "red",
							 NULL));
  glistener_set_prompt(g, prompt);
}

static const char *helper(glistener *g, const char *text)
{
  /* this function is called whenever the listener thinks help is needed.
   *   Any string it returns is posted in the listener statusbar.
   */
  s7_pointer sym;
  sym = s7_symbol_table_find_name(s7, text);
  if (sym)
    return(s7_help(s7, sym));
  glistener_clear_status(g);
  return(NULL);
}

static void completer(glistener *g, bool (*symbol_func)(const char *symbol_name, void *data), void *data)
{
  /* this function is called when &lt;tab&gt; is typed after a partial symbol name. 
   *   "symbol_func" above should be called on each member of the symbol-table, passing it
   *   the symbol name (as a string) and the data passed as "completer's" third argument.
   *   If symbol_func returns true, it is done, so the loop through the symbol-table can stop.
   */
  s7_for_each_symbol_name(s7, symbol_func, data);
}

int main(int argc, char **argv)
{
  GtkWidget *shell, *frame;
  glistener *g;

  s7 = s7_init();  

  gtk_init(&amp;argc, &amp;argv);
  shell = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  g_signal_connect(G_OBJECT(shell), "delete_event", G_CALLBACK(quit_repl), NULL);

  frame = gtk_frame_new(NULL);
  gtk_frame_set_shadow_type(GTK_FRAME(frame), GTK_SHADOW_ETCHED_IN);
  gtk_widget_show(frame);

  gtk_container_add(GTK_CONTAINER(shell), frame);

  /* make a new listener */
  g = glistener_new(frame, listener_init);
  glistener_set_evaluator(g, evaluator);
  glistener_set_helper(g, helper);
  glistener_set_completer(g, completer);

  gtk_widget_show(shell);
  gdk_window_resize(gtk_widget_get_window(shell), 400, 200);
  gtk_main();
}

/* in gtk-2: gcc gcall.c -o gcall s7.o glistener.o `pkg-config --libs gtk+-2.0 --cflags` -lm -ldl
 * in gtk-3: gcc gcall.c -o gcall s7.o glistener.o `pkg-config --libs gtk+-3.0 --cflags` -lm -ldl
 */
</pre>

<div class="listener">
<pre>
<em class=redb>&lambda;&gt;</em> (define &lambda; lambda)
<em class="gray">&lambda;</em>
<em class=redb>&lambda;&gt;</em> ((&lambda; (a b) (+ a b)) 1 2)
<em class="gray">3</em>
<em class=redb>&lambda;&gt;</em> 
</pre>
</div>

<br>
<p>The five or six functions supplied by the caller (evaluator, helper, 
completer, checker, colorizer, keyer) all have defaults, so you don't have to supply
anything but an evaluator.  The default evaluator just prints "?" and prompts
for more input.  See glistener.h for the full API and an earnest attempt
at helpful documentation.
</p>

<p>A multi-listener test program is the Snd file tools/gcall.c which
is used by tools/gtest.scm for regression testing. One way to name unicode characters
is: <code>(define-constant |lambda| #u8(#xce #xbb))</code>.  This can be embedded
in an ordinary s7 string with any string operation: <code>(string-append |lambda| "ambda")</code>
which returns "&lambda;ambda".  (string-length will still return the number of bytes; to get
the number of characters in a case like this, use g_utf8_strlen).
So, to set the prompt to be a red lambda and the font to be "Nimbus mono 10" from Scheme,
assuming we have the usual Scheme-to-C linkages (see snd-glistener.c):
</p>

<pre class="indented">
(set! (listener-prompt) (byte-vector #xce #xbb (char-&gt;integer #\&gt;) (char-&gt;integer #\space)))
(set! (listener-font) "Nimbus mono 10")
(listener-set-prompt-tag *listener* ; ideally this too would be a setter
  (gtk_text_buffer_create_tag 
    (GTK_TEXT_BUFFER (gtk_text_view_get_buffer (GTK_TEXT_VIEW (listener-text-widget *listener*))))
    "" (list "weight" PANGO_WEIGHT_BOLD "foreground" "red")))
</pre>

<p>In Snd, all the gtk code is in the *gtk* environment, so we need to use:
</p>
<pre class="indented">
(listener-set-prompt-tag *listener*
  (with-let (sublet *gtk* 'textw (listener-text-widget *listener*)) ; use *gtk*
    (gtk_text_buffer_create_tag 
      (GTK_TEXT_BUFFER (gtk_text_view_get_buffer (GTK_TEXT_VIEW textw)))
      "" (list "weight" PANGO_WEIGHT_BOLD "foreground" "red"))))
</pre>
</div>


<div class="header" id="gdb"><h4>gdb</h4></div>

<p>It is possible to make a mistake while writing C code.
I switched from Common Lisp to Scheme a long time ago
partly because it was so painful to debug FFI troubles in Common Lisp, and I
chose Guile at that time partly because I thought gdb would have native support
for it.  As far as I know it is still impossible to debug CL FFI troubles, 20 years later!
And in gdb Python has muscled Guile aside.  Anyway, say you have hit a segfault
and find yourself staring at a stackful of opaque pointers.  Print statements are your
friend, of course, and at the gdb command level, the main one in this context is
s7_object_to_c_string.  Here are some commands (intended for your .gdbinit file)
that can speed up the process.  They assume the s7_scheme pointer is named "sc".
(These are now included in the gdbinit file in the s7 tarball).
</p>

<pre class="indented">
define s7print
print s7_object_to_c_string(sc, $arg0)
end
document s7print
interpret the argument as an s7 value and display it
end
# the current expression is sc-&gt;cur_code
# the current environment is sc-&gt;envir
# the error environment is sc-&gt;owlet
# so for example, to see the current local variables, s7p sc-&gt;envir

define s7eval
print s7_object_to_c_string(sc, s7_eval_c_string(sc, $arg0))
end
document s7eval
eval the argument (a string)
end

define s7stack
print s7_object_to_c_string(sc, s7_stacktrace(sc))
end
document s7stack
display the current stack
end

define s7value
print s7_object_to_c_string(sc, s7_name_to_value(sc, $arg0))
end
document s7value
print the value of the variable passed by its print name: s7v "*features*"
end
</pre>

<p>gdbinit also has s7cell to decode every field of an s7_pointer, and two backtrace
decoders: s7bt and s7btfull (heh heh).  The bt replacements print the gdb backtrace info,
replacing bare pointer numbers with their s7 value, wherever possible:
</p>

<pre class="indented">
#1  0x000000000042104e in find_symbol_unchecked (sc=0x97edf0, symbol=<b>vars</b>) at s7.c:6677
        x = <b>(inlet 'f import-lambda-definition-2)</b>
        __FUNCTION__ = "find_symbol_unchecked"
#2  0x00000000006e3424 in eval (sc=0x97edf0, first_op=9) at s7.c:63673
        _x_ = <b>import-lambda-definition-2</b>
        _slot_ = <b>'form import-lambda-definition-2</b>
        _sym_ = <b>env</b>
        _val_ = <b>import-lambda-definition-2</b>
        args = <b>(vars)</b>
        p = <b>(env)</b>
        func = <b>lint-walk</b>
        e = <b>(inlet 'name import-lambda-definition-2 'form import-lambda-definition-2)</b>
        code = <b>(lint-walk name f vars)</b>
        __FUNCTION__ = "eval"
</pre>

<br><br>


<div class="topheader" id="s7examples">s7 examples</div>

<p>The s7 tarball includes several scheme files including s7test.scm,
lint.scm, cload.scm, write.scm, mockery.scm, and stuff.scm.  
s7test.scm is a regression test for s7, 
lint.scm is the s7 equivalent of the ancient C program named lint (modern equivalent: cppcheck), 
write.scm has a pretty printer, 
mockery.scm has mock data libraries,
cload.scm is a wrapper for the FFI stuff described above, and 
stuff.scm is just some arbitrary stuff.  
gdbinit has some gdb commands for s7. 
repl.scm is a repl.
profile.scm provides access to profiling data, if it's enabled.
</p>


<div class="header" id="cload"><h4>cload.scm</h4></div>

<p>cload.scm defines the macro c-define that reduces the overhead
involved in (dynamically) linking C entities into s7.
</p>

<pre class="indented">
(<em class=def id="definecfunction">c-define</em> c-info (prefix "") (headers ()) (cflags "") (ldflags "") output-name)
</pre>

<p>For example, <code>(c-define '(double j0 (double)) "m" "math.h")</code>
links the C math library function j0 into s7 under the name m:j0, 
passing it a double argument and getting a double result (a real in s7).
</p>

<p><em>prefix</em> is some arbitrary prefix that you want prepended to various names.
</p>

<p><em>headers</em> is a list of headers (as strings) that the c-info relies on, (("math.h") for example).
</p>

<p><em>cflags</em> are any special C compiler flags that are needed ("-I." in particular), and
<em>ldflags</em> is the similar case for the loader.  <em>output-name</em> is the name of the
output C file and associated library.  It defaults to "temp-s7-output" followed by a number.
In libm.scm, it is set to "libm_s7" to protect it across cload calls.  If cload finds an
up-to-date output C file and shared library, it simply loads the library, rather than
going through all the trouble of writing and compling it.
</p>

<p><em>c-info</em> is a list that describes the C entities that you want to load into s7.
It can be either one list describing one entity, or a list of such lists.
Each description has the form:
</p>

<pre class="indented">
(return-type entity-name-in-C (argument-type...))
</pre>

<p>where each entry is a symbol, and C names are used throughout.  So, in the j0
example above, <code>(double j0 (double))</code> says we want access to j0, it returns
a C double, and it takes one argument, also a C double.  s7 tries to figure out 
what the corresponding s7 type is, but in tricky cases, you should tell it
by replacing the bare type name with a list: <code>(C-type underlying-C-type)</code>.  For example,
the Snd function set_graph_style takes an (enum) argument of type graph_style_t.
This is actually an int, so we use <code>(graph_style_t int)</code> as the type:
</p>

<pre class="indented">
(void set_graph_style ((graph_style_t int)))
</pre>

<p>If the C entity is a constant, then the descriptor list has just two entries,
the C-type and the entity name: <code>(int F_OK)</code> for example. The entity name can also be a list:
</p>

<pre class="indented">
((graph_style_t int) (GRAPH_LINES GRAPH_DOTS GRAPH_FILLED GRAPH_DOTS_AND_LINES GRAPH_LOLLIPOPS))
</pre>

<p>This defines all the names in the list as integers.
If the C type has a space ("struct tm*"), use <code>(symbol "struct tm*")</code>
to construct the corresponding symbol.
</p>

<p>The entity is placed in the current s7 environment under the name <code>(string-append prefix ":" name)</code>
where the ":" is omitted if the prefix is null.  So in the j0 example, we get in s7 the function m:j0.
c-define returns #t if it thinks the load worked, and #f otherwise.
</p>

<p>There are times when the only straightforward approach is to write the desired
C code directly.  To insert C code on the fly, use (in-C "code..."). Two more such
cases that come up all the time: C-function for linkage to functions written
directly in s7 style using in-C, and C-macro for macros in the C header file that
need to be wrapped in #ifdefs.
Here are some examples:
</p>

<pre class="indented">
;;; various math library functions
(c-define '((double j0 (double)) 
            (double j1 (double)) 
            (double erf (double)) 
            (double erfc (double))
            (double lgamma (double)))
          "m" "math.h")


;;; getenv and setenv
(c-define '(char* getenv (char*)))
(c-define '(int setenv (char* char* int)))


;;; file-exists? and delete-file
(define file-exists? (let () ; define F_OK and access only within this let
                       (c-define '((int F_OK) (int access (char* int))) "" "unistd.h") 
                       (lambda (arg) (= (access arg F_OK) 0))))

(define delete-file (let () 
                      (c-define '(int unlink (char*)) "" "unistd.h") 
                      (lambda (file) (= (unlink file) 0)))) ; 0=success


;;; examples from Snd:
(c-define '(char* version_info ()) "" "snd.h" "-I.")

(c-define '(mus_float_t mus_degrees_to_radians (mus_float_t)) "" "snd.h" "-I.")

(c-define '(snd_info* any_selected_sound ()) "" "snd.h" "-I.")
(c-define '(void select_channel (snd_info* int)) "" "snd.h" "-I.")

(c-define '(((graph_style_t int) (GRAPH_LINES GRAPH_DOTS GRAPH_FILLED GRAPH_DOTS_AND_LINES GRAPH_LOLLIPOPS)) 
            (void set_graph_style ((graph_style_t int)))) 
          "" "snd.h" "-I.")
   

;;; getcwd, strftime
(c-define '(char* getcwd (char* size_t)) "" "unistd.h")

(c-define (list '(void* calloc (size_t size_t))
	        '(void free (void*))
	        '(void time (time_t*)) ; ignore returned value
	        (list (symbol "struct tm*") 'localtime '(time_t*))
                (list 'size_t 'strftime (list 'char* 'size_t 'char* (symbol "struct tm*"))))
          "" "time.h")

&gt; (let ((p (calloc 1 8)) 
        (str (make-string 32)))
    (time p) 
    (strftime str 32 "%a %d-%b-%Y %H:%M %Z" (localtime p))
    (free p) 
    str)
<em class="gray">"Sat 11-Aug-2012 08:55 PDT\x00      "</em>


;;; opendir, read_dir, closedir
(c-define '((int closedir (DIR*))
	    (DIR* opendir (char*))
	    (in-C "static char *read_dir(DIR *p)  \
                   {                              \
                     struct dirent *dirp;          \
                     dirp = readdir(p);            \
                     if (!dirp) return(NULL);      \
                     return(dirp-&gt;d_name);         \
                   }")
	    (char* read_dir (DIR*)))
  "" '("sys/types.h" "dirent.h"))

(let ((dir (opendir "/home/bil/gtk-snd")))
  (do ((p (read_dir dir) (read_dir dir)))
      ((= (length p) 0))
    (format *stderr* "~A " p))
  (closedir dir))
</pre>


<p>For the simple cases above, include "-ldl -Wl,-export-dynamic" in the gcc command.  So the first
FFI example is built (this is in Linux):
</p>

<pre class="indented">
gcc -c s7.c -I.
gcc -o ex1 ex1.c s7.o -lm -I. -ldl -Wl,-export-dynamic
ex1
&gt; (load "cload.scm")
<em class="gray">c-define-1</em>
&gt; (c-define '(double j0 (double)) "m" "math.h")
<em class="gray">#t</em>
&gt; (m:j0 0.5)
<em class="gray">0.93846980724081</em>
</pre>

<p>See also r7rs.scm, libc.scm, libgsl.scm, libm.scm, libdl.scm, and libgdbm.scm.
libutf8proc.scm exists, but I have not tested it at all.
</p>

<div class="indented" id="libc">
<pre>
(require libc.scm)

(define (copy-file in-file out-file)
  (with-let (sublet *libc* :in-file in-file :out-file out-file)

    ;; the rest of the function body exists in the *libc* environment, with the
    ;;   function parameters in-file and out-file imported, so, for example,
    ;;   (open ...) below calls the libc function open.  

    (let ((infd (open in-file O_RDONLY 0)))
      (if (= infd -1)
	  (error 'io-error "can't find ~S~%" in-file)
	  (let ((outfd (creat out-file #o666)))
	    (if (= outfd -1)
		(begin
		  (close infd)
		  (error 'io-error "can't open ~S~%" out-file))
		(let* ((BUF_SIZE 1024)
                       (buf (malloc BUF_SIZE)))
		  (do ((num (read infd buf BUF_SIZE) (read infd buf BUF_SIZE)))
		      ((or (&lt;= num 0)
			   (not (= (write outfd buf num) num)))))
		  (close outfd)
		  (close infd)
		  (free buf)
		  out-file)))))))

(define (glob-&gt;list pattern)
  (with-let (sublet *libc* :pattern pattern)
    (let ((g (glob.make))) 
      (glob pattern 0 g) 
      (let ((res (glob.gl_pathv g))) 
	(globfree g) 
	res))))

;; now (load "*.scm") is (for-each load (glob-&gt;list "*.scm")) 
</pre>
</div>


<div class="indented" id="libgsl">
<pre>
(require libgsl.scm)

(define (eigenvalues M)
  (with-let (sublet *libgsl* :M M)
    (let* ((len (sqrt (length M)))
	   (gm (gsl_matrix_alloc len len))
	   (m (float-vector-&gt;gsl_matrix M gm))
	   (evl (gsl_vector_complex_alloc len))
	   (evc (gsl_matrix_complex_alloc len len))
	   (w (gsl_eigen_nonsymmv_alloc len)))
      
      (gsl_eigen_nonsymmv m evl evc w)
      (gsl_eigen_nonsymmv_free w)
      (gsl_eigen_nonsymmv_sort evl evc GSL_EIGEN_SORT_ABS_DESC)
      
      (let ((vals (make-vector len)))
	(do ((i 0 (+ i 1)))
	    ((= i len))
	  (set! (vals i) (gsl_vector_complex_get evl i)))
	(gsl_matrix_free gm)
	(gsl_vector_complex_free evl)
	(gsl_matrix_complex_free evc)
	vals))))
</pre>
</div>

<p>We can use gdbm (or better yet, mdb), the :readable argument to object-&gt;string, and
the fallback methods in the environments to create name-spaces (lets) with billions of 
thread-safe local variables, which can be saved and communicated between s7 runs:
</p>
<div class="indented" id="libgdbm">
<pre>
(require libgdbm.scm)

(with-let *libgdbm*

  (define *db* 
    (openlet 
     (inlet :file (gdbm_open "test.gdbm" 1024 GDBM_NEWDB #o664 
		    (lambda (str) (format *stderr* "gdbm error: ~S~%" str)))

	    :let-ref-fallback (lambda (obj sym)
				(eval-string (gdbm_fetch (obj 'file) (symbol-&gt;string sym))))
	    
	    :let-set!-fallback (lambda (obj sym val)
				 (gdbm_store (obj 'file)
					     (symbol-&gt;string sym)
					     (object-&gt;string val :readable)
					     GDBM_REPLACE)
				 val)
	    
	    :make-iterator (lambda (obj)
			     (let ((key #f)
				   (length (lambda (obj) (expt 2 20))))
			       (#_make-iterator
                                (let ((iterator? #t))
				  (openlet 
				   (lambda ()
				     (if key
				         (set! key (gdbm_nextkey (obj 'file) (cdr key)))
				         (set! key (gdbm_firstkey (obj 'file))))
				     (if (pair? key)
				         (cons (string-&gt;symbol (car key))
					       (eval-string (gdbm_fetch (obj 'file) (car key))))
				         key))))))))))

  (set! (*db* 'str) "123") ; add a variable named 'str with the value "123"
  (set! (*db* 'int) 432)

  (with-let *db* 
    (+ int (length str)))    ; -&gt; 435
  (map values *db*)          ; -&gt; '((str . "123") (int . 432))

  (gdbm_close (*db* 'file)))
</pre>

</div>


<div class="header" id="schemerepl"><h4>repl.scm</h4></div>

<p>repl.scm implements a repl using vt100 codes and libc.scm.  It includes
symbol and filename completion, a history buffer, paren matching,
indentation, multi-line edits, and a debugger window.  
To move around in the history buffer, use M-p, M-n or M-. (C-p and C-n are used to move the cursor in the current expression).  
You can change the keymap or the prompt; all the repl functions are
accessible through the *repl* environment.  One field is 'repl-let which
gives you access to all the repl's internal variables and functions.
Another is 'top-level-let, normally (sublet (rootlet)), which is the environment in
which the repl's evaluation takes place.  You can reset the repl back to its
starting point with: <code>(set! (*repl* 'top-level-let) (sublet (rootlet)))</code>.
You can save the current repl state via <code>((*repl* 'save-repl))</code>, and
restore it later via <code>((*repl* 'restore-repl))</code>.  The repl's saved state
is in the file save.repl, or the filename can be passed as an argument to save-repl and restore-repl.
The special symbol '** holds the last value.
</p>

<p>Meta keys are a problem on the Mac.  You can use ESC instead, but that requires
super-human capacities.  I stared at replacement control keys, and nothing seemed
right.  If you can think of something, it's easy to define replacements: see repl.scm
which has a small table of mappings.
</p>

<p>To run the repl, either build s7 with the compiler flag -DWITH_MAIN,
or conjure up a wrapper:
</p>

<pre class="indented">
#include "s7.h"

int main(int argc, char **argv)
{
  s7_scheme *sc;
  sc = s7_init();
  s7_load(sc, "repl.scm");
  s7_eval_c_string(sc, "((*repl* 'run))");
  return(0);
}

/* gcc -o r r.c s7.o -Wl,-export-dynamic -lm -I. -ldl
 */
</pre>

<p>Besides evaluating s7 expressions, like any repl,
you can also type shell commands just as in a shell:
</p>

<pre class="indented">
&gt; pwd
<em class="gray">/home/bil/cl</em>
&gt; cd ..
<em class="gray">/home/bil</em>
&gt; date
<em class="gray">Wed 15-Apr-2015 17:32:24 PDT</em>
&gt; **
<em class="gray">"Wed 15-Apr-2015 17:32:24 PDT
"</em>
</pre>

<p>In most cases, these are handled through *unbound-variable-hook*, checked using "command -v", then passed
to the underlying shell via the system function.  If s7's (as opposed to libc's) system command
is accessible, the '** variable holds whatever the command printed.
</p>

<p>The prompt is set by the function (*repl* 'prompt).  It gets one argument,
the current line number, and should set the prompt string and its length.
</p>
<pre class="indented">
(set! (*repl* 'prompt) (lambda (num) 
			 (with-let (*repl* 'repl-let)
			   (set! prompt-string "scheme&gt; ") 
			   (set! prompt-length (length prompt-string)))))
</pre>
<p>or, to use the red lambda example mentioned earlier:
</p>
<pre class="indented">
(set! (*repl* 'prompt)
      (lambda (num)
	(with-let (*repl* 'repl-let)
	  (set! prompt-string (bold (red (string #\xce #\xbb #\&gt; #\space))))
	  (set! prompt-length 3)))) ; until we get unicode length calc
</pre>

<p>The line number provides a quick way to move around in the history buffer.
To get a previous line without laboriously typing M-p over and over,
simply type the line number (without control or meta bits), then M-.
</p>

<p>Here is an example of adding to the keymap:
</p>
<pre class="indented">
(set! ((*repl* 'keymap) (integer-&gt;char 17)) ; C-q to quit and return to caller
      (lambda (c)
	(set! ((*repl* 'repl-let) 'all-done) #t)))
</pre>

<p>To access the meta keys (in the keymap), use a string:
<code>((*repl* 'keymap) (string #\escape #\p))</code>; this is Meta-p which normally accesses
the history buffer.  
</p>

<p>You can call the repl from other code, poke around in the current environment (or whatever),
then return to the caller:
</p>

<pre class="indented">
(load "repl.scm")

(define (drop-into-repl e)
  (let ((C-q (integer-&gt;char 17)))              ; we'll use the C-q example above to get out
    (let ((old-C-q ((*repl* 'keymap) C-q))
	  (old-top-level (*repl* 'top-level-let)))
      (dynamic-wind
	  (lambda ()
	    (set! (*repl* 'top-level-let) e)
	    (set! ((*repl* 'keymap) C-q)       
		  (lambda (c)
		    (set! ((*repl* 'repl-let) 'all-done) #t))))
	  (lambda ()
	    ((<em class=red>*repl* 'run</em>)))                   ; run the repl
	  (lambda ()
	    (set! (*repl* 'top-level-let) old-top-level)
	    (set! ((*repl* 'keymap) C-q) old-C-q))))))

(let ((x 32))
  (format *stderr* "x: ~A~%" x)
  (<em class=red>drop-into-repl</em> (curlet))
  (format *stderr* "now x: ~A~%" x))
</pre>

<p>Now load that code and:
</p>

<pre class="indented">
x: 32
&gt; x
<em class="gray">32</em>
&gt; (set! x 91)
<em class="gray">91</em>
&gt; x
<em class="gray">91</em>
&gt; now x: 91  ; here I typed C-q at the prompt
</pre>

<p>Another possibility:
</p>
<pre class="indented">
(set! (hook-functions *error-hook*) 
      (list (lambda (hook) 
              (apply format *stderr* (hook 'data)) 
              (newline *stderr*)
	      (drop-into-repl (owlet)))))
</pre>

<p>See the end of repl.scm for more examples.
</p>

<!--
(load "/home/bil/test/sndlib/libsndlib.so" (inlet 'init_func 's7_init_sndlib))

not tested:
(load "/home/bil/test/libxm/libxm.so" (inlet 'init_func 'Init_libxm))

-->


<div class="header" id="lint"><h4>lint.scm</h4></div>

<p>lint tries to find errors or infelicities in your scheme code.
To try  it:
</p>

<pre class="indented">
(load "lint.scm")
(lint "some-code.scm")
</pre>


<p>
There are several
variables at the start of lint.scm to control additional output:
</p>


<pre class="indented">
*report-unused-parameters*
*report-unused-top-level-functions*
*report-shadowed-variables*
*report-undefined-identifiers*
*report-multiply-defined-top-level-functions*
*report-nested-if*
*report-short-branch*
*report-one-armed-if*
*report-loaded-files*
*report-any-!-as-setter*
*report-doc-strings*
*report-func-as-arg-arity-mismatch*
*report-constant-expressions-in-do*
*report-bad-variable-names*
*report-built-in-functions-used-as-variables*
*report-forward-functions*
*report-sloppy-assoc*
*report-bloated-arg*
</pre>

<p>See lint.scm for more about these switches.  You can also extend lint by adding your own code,
or adding your functions to lint's tables, or most simply by defining signatures for your functions.
snd-lint.scm performs these tasks for Snd.  (lint exports its innards via *lint*).
lint is not smart about functions defined outside the current file, so *report-undefined-variables*
sometimes gets confused. You'll sometimes get a recommendation from lint that is less than helpful; nobody's perfect.
If it's actually wrong, and not just wrong-headed, please let me know.
Also in lint.scm are html-lint and C-lint.  html-lint reads an HTML file looking for
Scheme code.  If any is found, it runs s7 and then lint over it, reporting troubles.
Similarly C-lint reads a C file looking for s7_eval_c_string and running lint over its string.
</p>


<blockquote>
<div class="indented">
<p>After months of intense typing,
Insanely declares his labors complete.  "Ship it!" says Mr Big, and hands
him a million stock options. Meanwhile, in the basement behind an old door
with the eldritch sign "eep Ou", in a labyrinth of pounding pipes and fluorescent lights,
a forgotten shadow types <code>(lint "insanely-great.scm")</code>...
</p>
</div>
</blockquote>


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