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<!DOCTYPE html>
<html lang="en">
<!-- documentation for s7 -->
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" >
<title>s7</title>
<style>
EM.red {color:red; font-style:normal}
EM.normal {font-style:normal}
EM.redb {color:red; font-weight: bold; font-style: normal}
EM.error {color:chocolate; font-style:normal}
EM.emdef {font-weight: bold; font-style: normal}
EM.green {color:green; font-style:normal}
EM.gray {color:#505050; font-style:normal}
EM.big {font-size: 20px; font-style: normal;}
EM.bigger {font-size: 30px; font-style: normal;}
EM.def {font-style: normal}
H1 {text-align: center}
UL {list-style-type: none}
A {text-decoration:none}
A:hover {text-decoration:underline}
A.def {font-weight: bold;
font-style: normal;
text-decoration:none;
}
PRE.indented {padding-left: 1.0cm;}
DIV.indented {background-color: #F8F8F0;
padding-left: 0.5cm;
padding-right: 0.5cm;
padding-top: 0.5cm;
padding-bottom: 0.5cm;
margin-bottom: 0.5cm;
border: 1px solid gray;
border-radius: 20px;
-moz-border-radius: 20px;
-webkit-border-radius: 20px;
}
DIV.small {font-size: small;
padding-left: 0.5cm;
padding-right: 0.5cm;
padding-bottom: 0.5cm;
}
DIV.header {margin-top: 60px;
margin-bottom: 30px;
border: 4px solid #00ff00; /* green */
background-color: #eefdee; /* lightgreen */
padding-left: 30px;
}
DIV.shortheader {margin-top: 30px;
margin-bottom: 10px;
border: 4px solid #00ff00; /* green */
background-color: #f5f5dc;
padding-left: 30px;
padding-top: 5px;
padding-bottom: 5px;
width: 20%;
}
DIV.topheader {margin-top: 10px;
margin-bottom: 40px;
border: 4px solid #00ff00; /* green */
background-color: #f5f5dc; /* beige */
font-family: 'Helvetica';
font-size: 30px;
text-align: center;
padding-top: 10px;
padding-bottom: 10px;
}
DIV.separator {margin-top: 30px;
margin-bottom: 10px;
border: 2px solid #00ff00; /* green */
background-color: #f5f5dc; /* beige */
padding-top: 4px;
width: 30%;
border-radius: 4px;
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
}
DIV.smallseparator {margin-top: 10px;
margin-bottom: 10px;
border: 2px solid #00ff00; /* green */
background-color: #f5f5dc; /* beige */
padding-top: 4px;
width: 20%;
border-radius: 4px;
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
text-align: center;
}
BODY.body {background-color: #ffffff; /* white */
margin-right: 20px;
margin-left: 20px;
}
DIV.listener {background-color: #f0f8ff;
font-family: 'Monospace';
padding-left: 6px;
padding-right: 6px;
padding-bottom: 4px;
margin-left: 1.0cm;
margin-right: 4.0cm;
border: 2px solid #a0a0a0;
}
LI.li_header {padding-top: 20px;}
</style>
</head>
<body class="body">
<!-- INDEX s7doc:s7 scheme -->
<div class="topheader" id="s7doc">s7
</div>
<p>s7 is a Scheme interpreter intended as an extension language for other applications.
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="s7-ffi.html#repl">repl</a>). s7test.scm is a regression test for s7.
A tarball is available: <a href="https://ccrma.stanford.edu/software/s7/s7.tar.gz">s7 tarball</a>.
There is an svn repository at sourceforge (the Snd project): <a href="https://sourceforge.net/p/snd/svn1/HEAD/tree/">Snd</a>,
and a git repository (just s7): [email protected]:bil/s7.git <a href="https://cm-gitlab.stanford.edu/bil/s7.git">s7.git</a>.
Please ignore all other "s7" github sites. Christos Vagias created a web-assembly site with
a repl: https://github.com/actonDev/s7-playground/.
</p>
<p>
s7 is an extension language of Snd and sndlib (<a href="https://ccrma.stanford.edu/software/snd/index.html">snd</a>),
Rick Taube's Common Music (commonmusic at sourceforge), Kjetil Matheussen's Radium music editor, and Iain Duncan's Scheme for Max (or Pd).
There are X, Motif, and openGL bindings
in libxm in the Snd tarball, or at ftp://ccrma-ftp.stanford.edu/pub/Lisp/libxm.tar.gz.
</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 thinks there is no 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 values.
</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>
<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">setter</a>
<li><a href="#generalizedset">generic functions, 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="#variableinfo">variable 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</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="#equivalentp">equivalent?</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="#legolambda">legolambda</a>, etc...
</ul>
<li class="li_header"><a href="s7-ffi.html#FFIexamples">FFI examples</a>
<ul>
<li><a href="s7-ffi.html#repl">read-eval-print loop (and emacs)</a>
<li><a href="s7-ffi.html#defun">define a function with arguments and a returned value, and define a variable </a>
<li><a href="s7-ffi.html#defvar">call a Scheme function from C, and get/set Scheme variable values in C</a>
<li><a href="s7-ffi.html#juce">C++ and Juce</a>
<li><a href="s7-ffi.html#sndlib">load sndlib using the Xen functions and macros</a>
<li><a href="s7-ffi.html#pwstype">add a new Scheme type and a procedure with a setter</a>
<li><a href="s7-ffi.html#functionportexample">redirect display output to a C procedure</a>
<li><a href="s7-ffi.html#extendop">extend a built-in operator ("+" in this case)</a>
<li><a href="s7-ffi.html#definestar1">C-side define* (s7_define_function_star)</a>
<li><a href="s7-ffi.html#definemacro1">C-side define-macro (s7_define_macro)</a>
<li><a href="s7-ffi.html#definegeneric">define a generic function in C</a>
<li><a href="s7-ffi.html#signal">signal handling (C-C to break out of an infinite loop)</a>
<li><a href="s7-ffi.html#notify">notification in C that a Scheme variable has been set!</a>
<li><a href="s7-ffi.html#namespace">Load C defined stuff into a separate namespace</a>
<li><a href="s7-ffi.html#Cerrors">Error handling in C</a>
<li><a href="s7-ffi.html#testhook">Hooks in C and Scheme</a>
<li><a href="s7-ffi.html#dload">Load a C module dynamically</a>
<li><a href="s7-ffi.html#gmpex">gmp and friends</a>
<li><a href="s7-ffi.html#gdb">gdb</a>
<li><a href="s7-ffi.html#webassembly">WASM</a>
<li><a href="s7-ffi.html#ffinotes">FFI notes</a>
</ul>
<li class="li_header"><a href="s7-scm.html#s7examples">s7 examples</a>
<ul>
<li><a href="s7-scm.html#cload">cload.scm</a>
<ul>
<li><a href="s7-scm.html#libc">libc</a>
<li><a href="s7-scm.html#libgsl">libgsl</a>
<li><a href="s7-scm.html#libgdbm">libgdbm</a>
</ul>
<li><a href="s7-scm.html#case">case.scm</a>
<li><a href="s7-scm.html#debug">debug.scm</a>
<li><a href="s7-scm.html#lint">lint.scm</a>
<li><a href="s7-scm.html#schemerepl">repl.scm and nrepl.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 int64_t and double.
A ratio consists of two integers, a complex number consists of two reals.
pi is predefined.
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">
> pi
<em class="gray">3.141592653589793238462643383279502884195E0</em>
> (*s7* 'bignum-precision)
<em class="gray">128</em>
> (set! (*s7* 'bignum-precision) 256)
<em class="gray">256</em>
> 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 either a number which it casts to a bignum, or a string representing the desired number:
</p>
<pre class="indented">
> (bignum "123456789123456789")
<em class="gray">123456789123456789</em>
> (bignum "1.123123123123123123123123123")
<em class="gray">1.12312312312312312312312312300000000009E0</em>
</pre>
<p>For read-time bignums:
</p>
<pre class="indented">
(set! *#readers*
(cons (cons #\B (lambda (str)
(bignum (string->number (substring str 1)))))
*#readers*))
</pre>
<p>and now #B123 is the reader equivalent of (bignum 123).
</p>
<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">
> (= 1.0 (+ 1.0 (expt 2.0 -128)))
<em class="gray">#t</em>
> (= 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">
> (= 1.0 (+ 1.0 (expt 2 -53)))
<em class="gray">#t</em>
> (= 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">
> (floor 1e56) ; (*s7* 'bignum-precision) is 128
<em class="gray">99999999999999999999999999999999999999927942405962072064</em>
> (set! (*s7* 'bignum-precision) 256)
<em class="gray">256</em>
> (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">
> (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-decode-float
<li>random
<li>nan?, infinite?, nan, nan-payload
</ul>
<p>
The random function can take any numeric argument, including 0.
Other math-related differences between s7 and r5rs:
</p>
<ul style="list-style-type:disc;">
<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. Is (= (+ #3# #20) ##5#)?
<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">
> (exact? 1.0)
<em class="gray">#f</em>
> (rational? 1.5)
<em class="gray">#f</em>
> (floor 1.4)
<em class="gray">1</em>
> (remainder 2.4 1)
<em class="gray">0.4</em>
> (modulo 1.4 1.0)
<em class="gray">0.4</em>
> (lcm 3/4 1/6)
<em class="gray">3/2</em>
> (log 8 2)
<em class="gray">3</em>
> (number->string 0.5 2)
<em class="gray">"0.1"</em>
> (string->number "0.1" 2)
<em class="gray">0.5</em>
> (rationalize 1.5)
<em class="gray">3/2</em>
> (complex 1/2 0)
<em class="gray">1/2</em>
> (logbit? 6 1) ; argument order, (logbit? int index), follows gmp, not CL
<em class="gray">#t</em>
</pre>
<p>See <a href="s7-scm.html#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->number "1e1" 16)</code> is ambiguous — is the "e" a digit or an exponent marker?
In s7, "@" is an exponent marker.
</p>
<pre class="indented">
> (string->number "1e9" 2) ; (expt 2 9)
<em class="gray">512.0</em>
> (string->number "1e1" 12) ; "e" is not a digit in base 12
<em class="gray">#f</em>
> (string->number "1e1" 16) ; (+ (* 1 16 16) (* 14 16) 1)
<em class="gray">481</em>
> (string->number "1.2e1" 3); (* 3 (+ 1 2/3))
<em class="gray">5.0</em>
</pre>
</div>
<div class="indented">
<p>
The functions <b>nan</b> and <b>nan-payload</b> refer to the "payload" that can be associated with a NaN. s7's reader
can read NaNs with these payloads: <code>+nan.123</code> is a NaN with payload 123. s7 displays the NaN payload
in the same way: +nan.123. A NaN without any payload (or payload 0) is written +nan.0.
</p>
<pre class="indented">
> (nan 123)
<em class="gray">+nan.123</em>
> (nan-payload (nan 123))
<em class="gray">123</em>
</pre>
<p>
The nan function (and s7's reader) always returns a positive NaN.
</p>
</div>
<div class="indented">
<p>
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 — 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:
</p>
<pre class="indented">
scheme@(guile-user)> (= -0.0 0.0)
<em class="gray">#t</em>
scheme@(guile-user)> (negative? -0.0)
<em class="gray">#f</em>
scheme@(guile-user)> (= (/ 1.0 0.0) (/ 1.0 -0.0))
<em class="gray">#f</em>
scheme@(guile-user)> (< (/ 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 seed is passed,
random-state returns the current state.
If no state is passed to random,
it 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.
</p>
<pre class="indented">
> (random 0)
<em class="gray">0</em>
> (random 1.0)
<em class="gray">0.86331198514245</em>
> (random 3/4)
<em class="gray">654/1129</em>
> (random 1+i)
<em class="gray">0.86300308872748+0.83601002730848i</em>
> (random -1.0)
<em class="gray">-0.037691127513267</em>
> (define r0 (random-state 1234))
<em class="gray">r0</em>
> (random 100 r0)
<em class="gray">94</em>
> (random 100 r0)
<em class="gray">19</em>
> (define r1 (random-state 1234))
<em class="gray">r1</em>
> (random 100 r1)
<em class="gray">94</em>
> (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->list, then to restart from that point, apply random-state to that list.
</p>
<p>In the gmp s7, random calls gmp's random number generator. There are
also many generators in GSL (see libgsl.scm). In the non-gmp s7, we use Marsaglia's MWC algorithm
which I think is a good compromise between quality and speed.
</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)> (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 —
its fractional part is explicitly zero! But +nan.0...
And then there's:
</p>
<pre class="indented">
(not-s7)> (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)> (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->exact:
</p>
<pre class="indented">
(not-s7)> (inexact->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->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 — 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->exact rationalize) ; or floor
(define (exact->inexact x) (* x 1.0))
</pre>
<p>Standard Scheme's #i and #e are also useless because you can
have any number after, for example, #b:
</p>
<pre class="indented">
> #b1.1
<em class="gray">1.5</em>
> #b1e2
<em class="gray">4.0</em>
> #o17.5+i
<em class="gray">15.625+1i</em>
</pre>
<p>s7 uses #i for int-vector and does not implement #e.
Speaking of #b and friends, what should <code>(string->number "#xffff" 2)</code> return?
</p>
</div>
</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 parameter 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 parameter name.
The last parameter
can be preceded by :rest or a dot to indicate that all other trailing arguments
should be packaged as a list under that parameter's name. A trailing or rest
parameter's default value is () and can't be specified in the declaration.
The rest parameter is not available as a keyword argument.
</p>
<pre class="indented">
(<em class=red>define*</em> (hi a (b 32) (c "hi")) (list a b c))
</pre>
<p>Here the parameter "a" defaults to #f, "b" to 32, etc.
When the function is called,
the parameter names are set from the values passed the function,
then any unset parameters 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 parameter'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">
> (hi 1)
<em class="gray">(1 32 "hi")</em>
> (hi :b 2 :a 3)
<em class="gray">(3 2 "hi")</em>
> (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*).
To mark an argument as required, set its default value to a call on the error function:
</p>
<pre class="indented">
> (define* (f a (b (error 'unset-arg "f's b parameter not set"))) (list a b))
<em class="gray">f</em>
> (f 1 2)
(1 2)
> (f 1)
<em class=red>error</em><em class="gray">: f's b parameter not set</em>
</pre>
<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">
> (let* func ((i 1) (j 2))
(+ i j (if (> i 0) (func (- i 1)) 0)))
<em class="gray">5</em>
> (letrec ((func (lambda* ((i 1) (j 2))
(+ i j (if (> 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">
> (defun foo (&key (a 0) (b (+ a 4)) (c (+ a 7))) (list a b c))
<em class="gray">FOO </em>
> (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>
<p>Also CL and s7 handle keywords as values in the same way:
</p>
<pre class="indented">
> (defun foo (&key a) a)
<em class="gray">FOO</em>
> (defvar x :a)
<em class="gray">X</em>
> (foo x 1)
<em class="gray">1</em>
</pre>
<pre class="indented">
> (define* (foo a) a)
<em class="gray">foo</em>
> (define x :a)
<em class="gray">:a</em>
> (foo x 1)
<em class="gray">1</em>
</pre>
<p>Keywords (named arguments) also work in named let*:
</p>
<pre class="indented">
> (let* loop ((i 0) (j 0))
(if (> i 3)
(+ i j)
(loop :j 2 :i (+ i 1))))
<em class="gray">6</em>
</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. To turn off these errors, add :allow-other-keys
at the end of the parameter list.
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>, or set <code>(*s7* 'accept-all-keyword-arguments)</code>
to some true value.
See s7test.scm for many examples.
</p>
<p>What if two functions share a keyword argument,
and one wants to call the other, passing both arguments to the wrapper?
</p>
<pre class="indented">
(define* (f1 a) a) ; the wrappee
(define* (f2 a :rest b <em class=red>:allow-other-keys</em>) ; the wrapper
(+ a (apply f1 b)))
(f2 :a 3 :a 4) ; 7, b='(:a 4)
(let ((c :a))
(f2 c 3 c 4)) ; also 7
</pre>
<p>Since named let* is a form of lambda*, the prohibition of repeated variable names makes it different
from let*: <code>(let* ((a 1) (a 2)) a)</code> is 2, but <code>(let* loop ((a 1) (a 2)) a)</code> is an error.
If let* and named let* agreed in this, we'd have an inconsistency with lambda*. If all three allowed repeated
variables, the decision as to which parameter is intended becomes messy: <code>((lambda* (a a) a) 2 :a 3)</code>,
or <code>(let* loop ((a 1) (a 2)) (loop 2 :a 3))</code>.
CL and standard scheme accept repeated variables in let*, so I think the current
choice is the least surprising.
</p>
</div>
<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">
> ((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">
> ((lambda* ((b 3) :rest x (c 1)) (list b c x)) 32)
<em class="gray">(32 1 ())</em>
> ((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 &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">
> ((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>
> ((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">
> (define* (f :rest a) a)
<em class="gray">f</em>
> (f :a 1)
<em class="gray">(:a 1)</em>
</pre>
</div>
<!--
unknown key handling is not good, :allow-key-values?
(define* (f1 a) (list a))
(f1 :hi) -> (:hi)
(f1 ':hi) -> (:hi)
(f1 :a :hi) -> (:hi)
(define* (f2 a :allow-other-keys) (list a))
(f2 :hi) -> (:hi)
(define* (f3 a b :allow-other-keys) (list a b))
(f3 :hi 0) -> (#f #f)
(f3 :hi) => (:hi #f)
(define* (f4 a) (list (symbol->keyword a))) ; or string->keyword
(f4 'hi) -> (:hi)
(define* (f5 a)
((lambda* (hi) (list hi)) (symbol->keyword a) 32)) ; -> ((lambda* (hi) (list hi)) :hi 32) -> '(32)
(f5 'hi) -> (32)
(define* (f6 a b)
((lambda* (hi) (list hi)) a b))
(f6 :hi 1) -> unknown key :hi -> (unknown to f6)
(define* (f7 a b) (list a b))
(f7 :hi) -> (:hi #f)
(f7 0 :hi) -> (0 :hi)
(f7 :hi 0) -> unknown key
(f7 :a :hi) -> (:hi #f)
(f7 :a :hi 32) -> (:hi 32)
(define* (f8 a b)
((lambda* (hi ho) (list hi ho)) (symbol->keyword a) b))
(f8 'hi 32) -> (32 #f)
(f8 'ho 32) -> (#f 32)
another amusing lambda* case:
(call/cc
(lambda* (a (b (call/cc (lambda (c) c)))) ; even with-baffle still a loop (legit)
(b (call/cc (lambda (d) d)))))
;; equivalent to:
(let ((c (call/cc (lambda (c) c))))
(call/cc (lambda (a)
(c (call/cc (lambda (d) d))))))
-->
</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="definebacro">define-bacro</em>,
<em class=def id="definebacrostar">define-bacro*</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.
The anonymous versions (analogous to lambda and lambda*) are
macro, macro*, bacro, and bacro*.
See s7test.scm for many examples of macros including such perennial favorites as
loop, dotimes, do*, enum, pushnew, and defstruct.
</p>
<pre class="indented">
> (define-macro (and-let* vars . body)
`(let ()
(and ,@(map (lambda (v)
`(define ,@v))
vars)
(begin ,@body))))
</pre>
<p>macroexpand can help debug a macro. I always forget that it
wants an expression:
</p>
<pre class="indented">
> (define-macro (add-1 arg) `(+ 1 ,arg))
<em class="gray">add-1</em>
> (macroexpand (add-1 32))
<em class="gray">(+ 1 32)</em>
</pre>
<p>gensym returns a symbol that is guaranteed to be unique. 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">
> (define-macro* (add-2 a (b 2)) `(+ ,a ,b))
<em class="gray">add-2</em>
> (add-2 1 3)
<em class="gray">4</em>
> (add-2 1)
<em class="gray">3</em>
> (add-2 :b 3 :a 1)
<em class="gray">4</em>
</pre>
<blockquote>
<div class="indented">
<p>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 setter!
</p>
<pre class="indented">
> (define-macro (hi a) `(+ ,a 1))
<em class="gray">hi</em>
> (apply hi '(4))
<em class="gray">5</em>
> (define (fmac mac) (apply mac '(4)))
<em class="gray">fmac</em>