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<a name="Scheme-Syntax-Extension-Packages-1"></a>
<h1 class="chapter">3. Scheme Syntax Extension Packages</h1>

<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Defmacro">3.1 Defmacro</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                    Supported by all implementations
</td></tr>
<tr><td align="left" valign="top"><a href="#R4RS-Macros">3.2 R4RS Macros</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                 &rsquo;macro
</td></tr>
<tr><td align="left" valign="top"><a href="#Macro-by-Example">3.3 Macro by Example</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">            &rsquo;macro-by-example
</td></tr>
<tr><td align="left" valign="top"><a href="#Macros-That-Work">3.4 Macros That Work</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">            &rsquo;macros-that-work
</td></tr>
<tr><td align="left" valign="top"><a href="#Syntactic-Closures">3.5 Syntactic Closures</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">          &rsquo;syntactic-closures
</td></tr>
<tr><td align="left" valign="top"><a href="#Syntax_002dCase-Macros">3.6 Syntax-Case Macros</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">          &rsquo;syntax-case
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Syntax extensions (macros) included with SLIB.

</pre></th></tr><tr><td align="left" valign="top"><a href="#Define_002dStructure">3.7 Define-Structure</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">            &rsquo;structure
</td></tr>
<tr><td align="left" valign="top"><a href="#Define_002dRecord_002dType">3.8 Define-Record-Type</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">          &rsquo;define-record-type, &rsquo;srfi-9
</td></tr>
<tr><td align="left" valign="top"><a href="#Fluid_002dLet">3.9 Fluid-Let</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                   &rsquo;fluid-let
</td></tr>
<tr><td align="left" valign="top"><a href="#Binding-to-multiple-values">3.10 Binding to multiple values</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">  &rsquo;receive, &rsquo;srfi-8
</td></tr>
<tr><td align="left" valign="top"><a href="#Guarded-LET_002a-special-form">3.11 Guarded LET* special form</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">   &rsquo;and-let*, &rsquo;srfi-2
</td></tr>
<tr><td align="left" valign="top"><a href="#Guarded-COND-Clause">3.12 Guarded COND Clause</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">         &rsquo;guarded-cond-clause, &rsquo;srfi-61
</td></tr>
<tr><td align="left" valign="top"><a href="#Yasos">3.13 Yasos</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                       &rsquo;yasos, &rsquo;oop, &rsquo;collect
</td></tr>
</table>

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<a name="Defmacro-1"></a>
<h2 class="section">3.1 Defmacro</h2>

<p>Defmacros are supported by all implementations.
</p>
<dl>
<dt><a name="index-gentemp"></a><u>Function:</u> <b>gentemp</b></dt>
<dd><p>Returns a new (interned) symbol each time it is called.  The symbol
names are implementation-dependent
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(gentemp) &rArr; scm:G0
(gentemp) &rArr; scm:G1
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-defmacro_003aeval"></a><u>Function:</u> <b>defmacro:eval</b><i> e</i></dt>
<dd><p>Returns the <code>slib:eval</code> of expanding all defmacros in scheme
expression <var>e</var>.
</p></dd></dl>

<dl>
<dt><a name="index-defmacro_003aload"></a><u>Function:</u> <b>defmacro:load</b><i> filename</i></dt>
<dd><p><var>filename</var> should be a string.  If filename names an existing file,
the <code>defmacro:load</code> procedure reads Scheme source code expressions
and definitions from the file and evaluates them sequentially.  These
source code expressions and definitions may contain defmacro
definitions.  The <code>macro:load</code> procedure does not affect the values
returned by <code>current-input-port</code> and
<code>current-output-port</code>.
</p></dd></dl>

<dl>
<dt><a name="index-defmacro_003f"></a><u>Function:</u> <b>defmacro?</b><i> sym</i></dt>
<dd><p>Returns <code>#t</code> if <var>sym</var> has been defined by <code>defmacro</code>,
<code>#f</code> otherwise.
</p></dd></dl>

<dl>
<dt><a name="index-macroexpand_002d1"></a><u>Function:</u> <b>macroexpand-1</b><i> form</i></dt>
<dt><a name="index-macroexpand"></a><u>Function:</u> <b>macroexpand</b><i> form</i></dt>
<dd><p>If <var>form</var> is a macro call, <code>macroexpand-1</code> will expand the
macro call once and return it.  A <var>form</var> is considered to be a macro
call only if it is a cons whose <code>car</code> is a symbol for which a
<code>defmacro</code> has been defined.
</p>
<p><code>macroexpand</code> is similar to <code>macroexpand-1</code>, but repeatedly
expands <var>form</var> until it is no longer a macro call.
</p></dd></dl>

<dl>
<dt><a name="index-defmacro-1"></a><u>Macro:</u> <b>defmacro</b><i> name lambda-list form &hellip;</i></dt>
<dd><p>When encountered by <code>defmacro:eval</code>, <code>defmacro:macroexpand*</code>,
or <code>defmacro:load</code> defines a new macro which will henceforth be
expanded when encountered by <code>defmacro:eval</code>,
<code>defmacro:macroexpand*</code>, or <code>defmacro:load</code>.
</p></dd></dl>

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<h3 class="subsection">3.1.1 Defmacroexpand</h3>
<p><code>(require 'defmacroexpand)</code>
<a name="index-defmacroexpand"></a>
</p>
<dl>
<dt><a name="index-defmacro_003aexpand_002a"></a><u>Function:</u> <b>defmacro:expand*</b><i> e</i></dt>
<dd><p>Returns the result of expanding all defmacros in scheme expression
<var>e</var>.
</p></dd></dl>


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<a name="R4RS-Macros-1"></a>
<h2 class="section">3.2 R4RS Macros</h2>

<p><code>(require 'macro)</code> is the appropriate call if you want R4RS
<a name="index-macro-1"></a>
high-level macros but don&rsquo;t care about the low level implementation.  If
an SLIB R4RS macro implementation is already loaded it will be used.
Otherwise, one of the R4RS macros implemetations is loaded.
</p>
<p>The SLIB R4RS macro implementations support the following uniform
interface:
</p>
<dl>
<dt><a name="index-macro_003aexpand"></a><u>Function:</u> <b>macro:expand</b><i> sexpression</i></dt>
<dd><p>Takes an R4RS expression, macro-expands it, and returns the result of
the macro expansion.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aeval"></a><u>Function:</u> <b>macro:eval</b><i> sexpression</i></dt>
<dd><p>Takes an R4RS expression, macro-expands it, evals the result of the
macro expansion, and returns the result of the evaluation.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aload"></a><u>Procedure:</u> <b>macro:load</b><i> filename</i></dt>
<dd><p><var>filename</var> should be a string.  If filename names an existing file,
the <code>macro:load</code> procedure reads Scheme source code expressions and
definitions from the file and evaluates them sequentially.  These source
code expressions and definitions may contain macro definitions.  The
<code>macro:load</code> procedure does not affect the values returned by
<code>current-input-port</code> and <code>current-output-port</code>.
</p></dd></dl>


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<a name="Macro-by-Example-1"></a>
<h2 class="section">3.3 Macro by Example</h2>

<p><code>(require 'macro-by-example)</code>
<a name="index-macro_002dby_002dexample-1"></a>
</p>
<p>A vanilla implementation of <cite>Macro by Example</cite> (Eugene Kohlbecker,
R4RS) by Dorai Sitaram, (dorai @ cs.rice.edu) using <code>defmacro</code>.
</p>
<ul>
<li>
generating hygienic global <code>define-syntax</code> Macro-by-Example macros
<strong>cheaply</strong>.

</li><li>
can define macros which use <code>...</code>.

</li><li>
needn&rsquo;t worry about a lexical variable in a macro definition
clashing with a variable from the macro use context

</li><li>
don&rsquo;t suffer the overhead of redefining the repl if <code>defmacro</code>
natively supported (most implementations)

</li></ul>
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<h3 class="subsection">3.3.1 Caveat</h3>
<p>These macros are not referentially transparent
(see <a href="../r4rs/Macros.html#Macros">(r4rs)Macros</a> section &lsquo;Macros&rsquo; in <cite>Revised(4) Scheme</cite>).  Lexically scoped macros
(i.e., <code>let-syntax</code> and <code>letrec-syntax</code>) are not supported.
In any case, the problem
of referential transparency gains poignancy only when <code>let-syntax</code>
and <code>letrec-syntax</code> are used.  So you will not be courting
large-scale disaster unless you&rsquo;re using system-function names as local
variables with unintuitive bindings that the macro can&rsquo;t use.  However,
if you must have the full <cite>r4rs</cite> macro functionality, look to the
more featureful (but also more expensive) versions of syntax-rules
available in slib <a href="#Macros-That-Work">Macros That Work</a>, <a href="#Syntactic-Closures">Syntactic Closures</a>, and
<a href="#Syntax_002dCase-Macros">Syntax-Case Macros</a>.
</p>
<dl>
<dt><a name="index-define_002dsyntax"></a><u>Macro:</u> <b>define-syntax</b><i> keyword transformer-spec</i></dt>
<dd><p>The <var>keyword</var> is an identifier, and the <var>transformer-spec</var>
should be an instance of <code>syntax-rules</code>.
</p>
<p>The top-level syntactic environment is extended by binding the
<var>keyword</var> to the specified transformer.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">(define-syntax let*
  (syntax-rules ()
    ((let* () body1 body2 ...)
     (let () body1 body2 ...))
    ((let* ((name1 val1) (name2 val2) ...)
       body1 body2 ...)
     (let ((name1 val1))
       (let* (( name2 val2) ...)
         body1 body2 ...)))))
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-syntax_002drules"></a><u>Macro:</u> <b>syntax-rules</b><i> literals syntax-rule &hellip;</i></dt>
<dd><p><var>literals</var> is a list of identifiers, and each <var>syntax-rule</var>
should be of the form
</p>
<p><code>(<var>pattern</var> <var>template</var>)</code>
</p>
<p>where the <var>pattern</var> and  <var>template</var> are as in the grammar above.
</p>
<p>An instance of <code>syntax-rules</code> produces a new macro transformer by
specifying a sequence of hygienic rewrite rules.  A use of a macro whose
keyword is associated with a transformer specified by
<code>syntax-rules</code> is matched against the patterns contained in the
<var>syntax-rule</var>s, beginning with the leftmost <var>syntax-rule</var>.
When a match is found, the macro use is trancribed hygienically
according to the template.
</p>
<p>Each pattern begins with the keyword for the macro.  This keyword is not
involved in the matching and is not considered a pattern variable or
literal identifier.
</p></dd></dl>


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<a name="Macros-That-Work-1"></a>
<h2 class="section">3.4 Macros That Work</h2>

<p><code>(require 'macros-that-work)</code>
<a name="index-macros_002dthat_002dwork-1"></a>
</p>
<p><cite>Macros That Work</cite> differs from the other R4RS macro
implementations in that it does not expand derived expression types to
primitive expression types.
</p>
<dl>
<dt><a name="index-macro_003aexpand-1"></a><u>Function:</u> <b>macro:expand</b><i> expression</i></dt>
<dt><a name="index-macwork_003aexpand"></a><u>Function:</u> <b>macwork:expand</b><i> expression</i></dt>
<dd><p>Takes an R4RS expression, macro-expands it, and returns the result of
the macro expansion.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aeval-1"></a><u>Function:</u> <b>macro:eval</b><i> expression</i></dt>
<dt><a name="index-macwork_003aeval"></a><u>Function:</u> <b>macwork:eval</b><i> expression</i></dt>
<dd><p><code>macro:eval</code> returns the value of <var>expression</var> in the current
top level environment.  <var>expression</var> can contain macro definitions.
Side effects of <var>expression</var> will affect the top level
environment.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aload-1"></a><u>Procedure:</u> <b>macro:load</b><i> filename</i></dt>
<dt><a name="index-macwork_003aload"></a><u>Procedure:</u> <b>macwork:load</b><i> filename</i></dt>
<dd><p><var>filename</var> should be a string.  If filename names an existing file,
the <code>macro:load</code> procedure reads Scheme source code expressions and
definitions from the file and evaluates them sequentially.  These source
code expressions and definitions may contain macro definitions.  The
<code>macro:load</code> procedure does not affect the values returned by
<code>current-input-port</code> and <code>current-output-port</code>.
</p></dd></dl>

<p>References:
</p>
<p>The <cite>Revised^4 Report on the Algorithmic Language Scheme</cite> Clinger
and Rees [editors].  To appear in LISP Pointers.  Also available as a
technical report from the University of Oregon, MIT AI Lab, and
Cornell.
</p>
<p align="center"> Macros That Work.  Clinger and Rees.  POPL &rsquo;91.
</p>
<p>The supported syntax differs from the R4RS in that vectors are allowed
as patterns and as templates and are not allowed as pattern or template
data.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">transformer spec  &rarr;  (syntax-rules literals rules)

rules  &rarr;  ()
         |  (rule . rules)

rule  &rarr;  (pattern template)

pattern  &rarr;  pattern_var      ; a symbol not in literals
           |  symbol           ; a symbol in literals
           |  ()
           |  (pattern . pattern)
           |  (ellipsis_pattern)
           |  #(pattern*)                     ; extends R4RS
           |  #(pattern* ellipsis_pattern)    ; extends R4RS
           |  pattern_datum

template  &rarr;  pattern_var
            |  symbol
            |  ()
            |  (template2 . template2)
            |  #(template*)                   ; extends R4RS
            |  pattern_datum

template2  &rarr;  template
             |  ellipsis_template

pattern_datum  &rarr;  string                    ; no vector
                 |  character
                 |  boolean
                 |  number

ellipsis_pattern  &rarr; pattern ...

ellipsis_template  &rarr;  template ...

pattern_var  &rarr;  symbol   ; not in literals

literals  &rarr;  ()
            |  (symbol . literals)
</pre></td></tr></table>

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<h3 class="subsection">3.4.1 Definitions</h3>

<dl compact="compact">
<dt> Scope of an ellipsis</dt>
<dd><p>Within a pattern or template, the scope of an ellipsis (<code>...</code>) is
the pattern or template that appears to its left.
</p>
</dd>
<dt> Rank of a pattern variable</dt>
<dd><p>The rank of a pattern variable is the number of ellipses within whose
scope it appears in the pattern.
</p>
</dd>
<dt> Rank of a subtemplate</dt>
<dd><p>The rank of a subtemplate is the number of ellipses within whose scope
it appears in the template.
</p>
</dd>
<dt> Template rank of an occurrence of a pattern variable</dt>
<dd><p>The template rank of an occurrence of a pattern variable within a
template is the rank of that occurrence, viewed as a subtemplate.
</p>
</dd>
<dt> Variables bound by a pattern</dt>
<dd><p>The variables bound by a pattern are the pattern variables that appear
within it.
</p>
</dd>
<dt> Referenced variables of a subtemplate</dt>
<dd><p>The referenced variables of a subtemplate are the pattern variables that
appear within it.
</p>
</dd>
<dt> Variables opened by an ellipsis template</dt>
<dd><p>The variables opened by an ellipsis template are the referenced pattern
variables whose rank is greater than the rank of the ellipsis template.
</p>
</dd>
</dl>

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<h3 class="subsection">3.4.2 Restrictions</h3>

<p>No pattern variable appears more than once within a pattern.
</p>
<p>For every occurrence of a pattern variable within a template, the
template rank of the occurrence must be greater than or equal to the
pattern variable&rsquo;s rank.
</p>
<p>Every ellipsis template must open at least one variable.
</p>
<p>For every ellipsis template, the variables opened by an ellipsis
template must all be bound to sequences of the same length.
</p>
<p>The compiled form of a <var>rule</var> is
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">rule  &rarr;  (pattern template inserted)

pattern  &rarr;  pattern_var
           |  symbol
           |  ()
           |  (pattern . pattern)
           |  ellipsis_pattern
           |  #(pattern)
           |  pattern_datum

template  &rarr;  pattern_var
            |  symbol
            |  ()
            |  (template2 . template2)
            |  #(pattern)
            |  pattern_datum

template2  &rarr;  template
             |  ellipsis_template

pattern_datum  &rarr;  string
                 |  character
                 |  boolean
                 |  number

pattern_var  &rarr;  #(V symbol rank)

ellipsis_pattern  &rarr;  #(E pattern pattern_vars)

ellipsis_template  &rarr;  #(E template pattern_vars)

inserted  &rarr;  ()
            |  (symbol . inserted)

pattern_vars  &rarr;  ()
                |  (pattern_var . pattern_vars)

rank  &rarr;  exact non-negative integer
</pre></td></tr></table>

<p>where V and E are unforgeable values.
</p>
<p>The pattern variables associated with an ellipsis pattern are the
variables bound by the pattern, and the pattern variables associated
with an ellipsis template are the variables opened by the ellipsis
template.
</p>
<p>If the template contains a big chunk that contains no pattern variables
or inserted identifiers, then the big chunk will be copied
unnecessarily.  That shouldn&rsquo;t matter very often.
</p>

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<a name="Syntactic-Closures-1"></a>
<h2 class="section">3.5 Syntactic Closures</h2>

<p><code>(require 'syntactic-closures)</code>
<a name="index-syntactic_002dclosures-1"></a>
</p>
<dl>
<dt><a name="index-macro_003aexpand-2"></a><u>Function:</u> <b>macro:expand</b><i> expression</i></dt>
<dt><a name="index-synclo_003aexpand"></a><u>Function:</u> <b>synclo:expand</b><i> expression</i></dt>
<dd><p>Returns scheme code with the macros and derived expression types of
<var>expression</var> expanded to primitive expression types.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aeval-2"></a><u>Function:</u> <b>macro:eval</b><i> expression</i></dt>
<dt><a name="index-synclo_003aeval"></a><u>Function:</u> <b>synclo:eval</b><i> expression</i></dt>
<dd><p><code>macro:eval</code> returns the value of <var>expression</var> in the current
top level environment.  <var>expression</var> can contain macro definitions.
Side effects of <var>expression</var> will affect the top level
environment.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aload-2"></a><u>Procedure:</u> <b>macro:load</b><i> filename</i></dt>
<dt><a name="index-synclo_003aload"></a><u>Procedure:</u> <b>synclo:load</b><i> filename</i></dt>
<dd><p><var>filename</var> should be a string.  If filename names an existing file,
the <code>macro:load</code> procedure reads Scheme source code expressions and
definitions from the file and evaluates them sequentially.  These
source code expressions and definitions may contain macro definitions.
The <code>macro:load</code> procedure does not affect the values returned by
<code>current-input-port</code> and <code>current-output-port</code>.
</p></dd></dl>

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<h3 class="subsection">3.5.1 Syntactic Closure Macro Facility</h3>

<p align="center"> A Syntactic Closures Macro Facility
</p><p align="center"> by Chris Hanson
</p><p align="center"> 9 November 1991
</p>
<p>This document describes <em>syntactic closures</em>, a low-level macro
facility for the Scheme programming language.  The facility is an
alternative to the low-level macro facility described in the
<cite>Revised^4 Report on Scheme.</cite> This document is an addendum to that
report.
</p>
<p>The syntactic closures facility extends the BNF rule for
<var>transformer spec</var> to allow a new keyword that introduces a
low-level macro transformer:
</p>
<table><tr><td>&nbsp;</td><td><pre class="example"><var>transformer spec</var> := (transformer <var>expression</var>)
</pre></td></tr></table>

<p>Additionally, the following procedures are added:
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">make-syntactic-closure
capture-syntactic-environment
identifier?
identifier=?
</pre></td></tr></table>

<p>The description of the facility is divided into three parts.  The first
part defines basic terminology.  The second part describes how macro
transformers are defined.  The third part describes the use of
<em>identifiers</em>, which extend the syntactic closure mechanism to be
compatible with <code>syntax-rules</code>.
</p>
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<h4 class="subsubsection">3.5.1.1 Terminology</h4>

<p>This section defines the concepts and data types used by the syntactic
closures facility.
</p>
<ul>
<li> <em>Forms</em> are the syntactic entities out of which programs are
recursively constructed.  A form is any expression, any definition, any
syntactic keyword, or any syntactic closure.  The variable name that
appears in a <code>set!</code> special form is also a form.  Examples of
forms:

<table><tr><td>&nbsp;</td><td><pre class="lisp">17
#t
car
(+ x 4)
(lambda (x) x)
(define pi 3.14159)
if
define
</pre></td></tr></table>

</li><li> An <em>alias</em> is an alternate name for a given symbol.  It can
appear anywhere in a form that the symbol could be used, and when quoted
it is replaced by the symbol; however, it does not satisfy the predicate
<code>symbol?</code>.  Macro transformers rarely distinguish symbols from
aliases, referring to both as identifiers.

</li><li> A <em>syntactic</em> environment maps identifiers to their
meanings.  More precisely, it determines whether an identifier is a
syntactic keyword or a variable.  If it is a keyword, the meaning is an
interpretation for the form in which that keyword appears.  If it is a
variable, the meaning identifies which binding of that variable is
referenced.  In short, syntactic environments contain all of the
contextual information necessary for interpreting the meaning of a
particular form.

</li><li> A <em>syntactic closure</em> consists of a form, a syntactic
environment, and a list of identifiers.  All identifiers in the form
take their meaning from the syntactic environment, except those in the
given list.  The identifiers in the list are to have their meanings
determined later.  A syntactic closure may be used in any context in
which its form could have been used.  Since a syntactic closure is also
a form, it may not be used in contexts where a form would be illegal.
For example, a form may not appear as a clause in the cond special form.
A syntactic closure appearing in a quoted structure is replaced by its
form.

</li></ul>

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<h4 class="subsubsection">3.5.1.2 Transformer Definition</h4>

<p>This section describes the <code>transformer</code> special form and the
procedures <code>make-syntactic-closure</code> and
<code>capture-syntactic-environment</code>.
</p>
<dl>
<dt><a name="index-transformer"></a><u>Syntax:</u> <b>transformer</b><i> expression</i></dt>
<dd>
<p>Syntax: It is an error if this syntax occurs except as a
<var>transformer spec</var>.
</p>
<p>Semantics: The <var>expression</var> is evaluated in the standard transformer
environment to yield a macro transformer as described below.  This macro
transformer is bound to a macro keyword by the special form in which the
<code>transformer</code> expression appears (for example,
<code>let-syntax</code>).
</p>
<p>A <em>macro transformer</em> is a procedure that takes two arguments, a
form and a syntactic environment, and returns a new form.  The first
argument, the <em>input form</em>, is the form in which the macro keyword
occurred.  The second argument, the <em>usage environment</em>, is the
syntactic environment in which the input form occurred.  The result of
the transformer, the <em>output form</em>, is automatically closed in the
<em>transformer environment</em>, which is the syntactic environment in
which the <code>transformer</code> expression occurred.
</p>
<p>For example, here is a definition of a push macro using
<code>syntax-rules</code>:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax  push
  (syntax-rules ()
    ((push item list)
     (set! list (cons item list)))))
</pre></td></tr></table>

<p>Here is an equivalent definition using <code>transformer</code>:
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax push
  (transformer
   (lambda (exp env)
     (let ((item
            (make-syntactic-closure env '() (cadr exp)))
           (list
            (make-syntactic-closure env '() (caddr exp))))
       `(set! ,list (cons ,item ,list))))))
</pre></td></tr></table>

<p>In this example, the identifiers <code>set!</code> and <code>cons</code> are closed
in the transformer environment, and thus will not be affected by the
meanings of those identifiers in the usage environment
<code>env</code>.
</p>
<p>Some macros may be non-hygienic by design.  For example, the following
defines a loop macro that implicitly binds <code>exit</code> to an escape
procedure.  The binding of <code>exit</code> is intended to capture free
references to <code>exit</code> in the body of the loop, so <code>exit</code> must
be left free when the body is closed:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax loop
  (transformer
   (lambda (exp env)
     (let ((body (cdr exp)))
       `(call-with-current-continuation
         (lambda (exit)
           (let f ()
             ,@(map (lambda  (exp)
                       (make-syntactic-closure env '(exit)
                                               exp))
                     body)
             (f))))))))
</pre></td></tr></table>

<p>To assign meanings to the identifiers in a form, use
<code>make-syntactic-closure</code> to close the form in a syntactic
environment.
</p></dd></dl>

<dl>
<dt><a name="index-make_002dsyntactic_002dclosure"></a><u>Function:</u> <b>make-syntactic-closure</b><i> environment free-names form</i></dt>
<dd>
<p><var>environment</var> must be a syntactic environment, <var>free-names</var> must
be a list of identifiers, and <var>form</var> must be a form.
<code>make-syntactic-closure</code> constructs and returns a syntactic closure
of <var>form</var> in <var>environment</var>, which can be used anywhere that
<var>form</var> could have been used.  All the identifiers used in
<var>form</var>, except those explicitly excepted by <var>free-names</var>, obtain
their meanings from <var>environment</var>.
</p>
<p>Here is an example where <var>free-names</var> is something other than the
empty list.  It is instructive to compare the use of <var>free-names</var> in
this example with its use in the <code>loop</code> example above: the examples
are similar except for the source of the identifier being left
free.
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax let1
  (transformer
   (lambda (exp env)
     (let ((id (cadr exp))
           (init (caddr exp))
           (exp (cadddr exp)))
       `((lambda (,id)
           ,(make-syntactic-closure env (list id) exp))
         ,(make-syntactic-closure env '() init))))))
</pre></td></tr></table>

<p><code>let1</code> is a simplified version of <code>let</code> that only binds a
single identifier, and whose body consists of a single expression.  When
the body expression is syntactically closed in its original syntactic
environment, the identifier that is to be bound by <code>let1</code> must be
left free, so that it can be properly captured by the <code>lambda</code> in
the output form.
</p>
<p>To obtain a syntactic environment other than the usage environment, use
<code>capture-syntactic-environment</code>.
</p></dd></dl>

<dl>
<dt><a name="index-capture_002dsyntactic_002denvironment"></a><u>Function:</u> <b>capture-syntactic-environment</b><i> procedure</i></dt>
<dd>
<p><code>capture-syntactic-environment</code> returns a form that will, when
transformed, call <var>procedure</var> on the current syntactic environment.
<var>procedure</var> should compute and return a new form to be transformed,
in that same syntactic environment, in place of the form.
</p>
<p>An example will make this clear.  Suppose we wanted to define a simple
<code>loop-until</code> keyword equivalent to
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax loop-until
  (syntax-rules ()
    ((loop-until id init test return step)
     (letrec ((loop
               (lambda (id)
                 (if test return (loop step)))))
       (loop init)))))
</pre></td></tr></table>

<p>The following attempt at defining <code>loop-until</code> has a subtle bug:
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax loop-until
  (transformer
   (lambda (exp env)
     (let ((id (cadr exp))
           (init (caddr exp))
           (test (cadddr exp))
           (return (cadddr (cdr exp)))
           (step (cadddr (cddr exp)))
           (close
            (lambda (exp free)
              (make-syntactic-closure env free exp))))
       `(letrec ((loop
                  (lambda (,id)
                    (if ,(close test (list id))
                        ,(close return (list id))
                        (loop ,(close step (list id)))))))
          (loop ,(close init '())))))))
</pre></td></tr></table>

<p>This definition appears to take all of the proper precautions to prevent
unintended captures.  It carefully closes the subexpressions in their
original syntactic environment and it leaves the <code>id</code> identifier
free in the <code>test</code>, <code>return</code>, and <code>step</code> expressions, so
that it will be captured by the binding introduced by the <code>lambda</code>
expression.  Unfortunately it uses the identifiers <code>if</code> and
<code>loop</code> within that <code>lambda</code> expression, so if the user of
<code>loop-until</code> just happens to use, say, <code>if</code> for the
identifier, it will be inadvertently captured.
</p>
<p>The syntactic environment that <code>if</code> and <code>loop</code> want to be
exposed to is the one just outside the <code>lambda</code> expression: before
the user&rsquo;s identifier is added to the syntactic environment, but after
the identifier loop has been added.
<code>capture-syntactic-environment</code> captures exactly that environment
as follows:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(define-syntax loop-until
  (transformer
   (lambda (exp env)
     (let ((id (cadr exp))
           (init (caddr exp))
           (test (cadddr exp))
           (return (cadddr (cdr exp)))
           (step (cadddr (cddr exp)))
           (close
            (lambda (exp free)
              (make-syntactic-closure env free exp))))
       `(letrec ((loop
                  ,(capture-syntactic-environment
                    (lambda (env)
                      `(lambda (,id)
                         (,(make-syntactic-closure env '() `if)
                          ,(close test (list id))
                          ,(close return (list id))
                          (,(make-syntactic-closure env '()
                                                    `loop)
                           ,(close step (list id)))))))))
          (loop ,(close init '())))))))
</pre></td></tr></table>

<p>In this case, having captured the desired syntactic environment, it is
convenient to construct syntactic closures of the identifiers <code>if</code>
and the <code>loop</code> and use them in the body of the
<code>lambda</code>.
</p>
<p>A common use of <code>capture-syntactic-environment</code> is to get the
transformer environment of a macro transformer:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(transformer
 (lambda (exp env)
   (capture-syntactic-environment
    (lambda (transformer-env)
      ...))))
</pre></td></tr></table>
</dd></dl>

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<h4 class="subsubsection">3.5.1.3 Identifiers</h4>

<p>This section describes the procedures that create and manipulate
identifiers.  Previous syntactic closure proposals did not have an
identifier data type &ndash; they just used symbols.  The identifier data
type extends the syntactic closures facility to be compatible with the
high-level <code>syntax-rules</code> facility.
</p>
<p>As discussed earlier, an identifier is either a symbol or an
<em>alias</em>.  An alias is implemented as a syntactic closure whose
<em>form</em> is an identifier:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(make-syntactic-closure env '() 'a)
   &rArr; an <em>alias</em>
</pre></td></tr></table>

<p>Aliases are implemented as syntactic closures because they behave just
like syntactic closures most of the time.  The difference is that an
alias may be bound to a new value (for example by <code>lambda</code> or
<code>let-syntax</code>); other syntactic closures may not be used this way.
If an alias is bound, then within the scope of that binding it is looked
up in the syntactic environment just like any other identifier.
</p>
<p>Aliases are used in the implementation of the high-level facility
<code>syntax-rules</code>.  A macro transformer created by <code>syntax-rules</code>
uses a template to generate its output form, substituting subforms of
the input form into the template.  In a syntactic closures
implementation, all of the symbols in the template are replaced by
aliases closed in the transformer environment, while the output form
itself is closed in the usage environment.  This guarantees that the
macro transformation is hygienic, without requiring the transformer to
know the syntactic roles of the substituted input subforms.
</p>
<dl>
<dt><a name="index-identifier_003f"></a><u>Function:</u> <b>identifier?</b><i>  object</i></dt>
<dd><p>Returns <code>#t</code> if <var>object</var> is an identifier, otherwise returns
<code>#f</code>.  Examples:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(identifier? 'a)
   &rArr; #t
(identifier? (make-syntactic-closure env '() 'a))
   &rArr; #t
(identifier? &quot;a&quot;)
   &rArr; #f
(identifier? #\a)
   &rArr; #f
(identifier? 97)
   &rArr; #f
(identifier? #f)
   &rArr; #f
(identifier? '(a))
   &rArr; #f
(identifier? '#(a))
   &rArr; #f
</pre></td></tr></table>

<p>The predicate <code>eq?</code> is used to determine if two identifers are
&ldquo;the same&rdquo;.  Thus <code>eq?</code> can be used to compare identifiers
exactly as it would be used to compare symbols.  Often, though, it is
useful to know whether two identifiers &ldquo;mean the same thing&rdquo;.  For
example, the <code>cond</code> macro uses the symbol <code>else</code> to identify
the final clause in the conditional.  A macro transformer for
<code>cond</code> cannot just look for the symbol <code>else</code>, because the
<code>cond</code> form might be the output of another macro transformer that
replaced the symbol <code>else</code> with an alias.  Instead the transformer
must look for an identifier that &ldquo;means the same thing&rdquo; in the usage
environment as the symbol <code>else</code> means in the transformer
environment.
</p></dd></dl>

<dl>
<dt><a name="index-identifier_003d_003f"></a><u>Function:</u> <b>identifier=?</b><i> environment1 identifier1 environment2 identifier2</i></dt>
<dd><p><var>environment1</var> and <var>environment2</var> must be syntactic
environments, and <var>identifier1</var> and <var>identifier2</var> must be
identifiers.  <code>identifier=?</code> returns <code>#t</code> if the meaning of
<var>identifier1</var> in <var>environment1</var> is the same as that of
<var>identifier2</var> in <var>environment2</var>, otherwise it returns <code>#f</code>.
Examples:
</p>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(let-syntax
    ((foo
      (transformer
       (lambda (form env)
         (capture-syntactic-environment
          (lambda (transformer-env)
            (identifier=? transformer-env 'x env 'x)))))))
  (list (foo)
        (let ((x 3))
          (foo))))
   &rArr; (#t #f)
</pre></td></tr></table>

<table><tr><td>&nbsp;</td><td><pre class="lisp">(let-syntax ((bar foo))
  (let-syntax
      ((foo
        (transformer
         (lambda (form env)
           (capture-syntactic-environment
            (lambda (transformer-env)
              (identifier=? transformer-env 'foo
                            env (cadr form))))))))
    (list (foo foo)
          (foobar))))
   &rArr; (#f #t)
</pre></td></tr></table>
</dd></dl>

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<h4 class="subsubsection">3.5.1.4 Acknowledgements</h4>

<p>The syntactic closures facility was invented by Alan Bawden and Jonathan
Rees.  The use of aliases to implement <code>syntax-rules</code> was invented
by Alan Bawden (who prefers to call them <em>synthetic names</em>).  Much
of this proposal is derived from an earlier proposal by Alan
Bawden.
</p>

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<a name="Syntax_002dCase-Macros-1"></a>
<h2 class="section">3.6 Syntax-Case Macros</h2>

<p><code>(require 'syntax-case)</code>
<a name="index-syntax_002dcase-1"></a>
</p>
<dl>
<dt><a name="index-macro_003aexpand-3"></a><u>Function:</u> <b>macro:expand</b><i> expression</i></dt>
<dt><a name="index-syncase_003aexpand"></a><u>Function:</u> <b>syncase:expand</b><i> expression</i></dt>
<dd><p>Returns scheme code with the macros and derived expression types of
<var>expression</var> expanded to primitive expression types.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aeval-3"></a><u>Function:</u> <b>macro:eval</b><i> expression</i></dt>
<dt><a name="index-syncase_003aeval"></a><u>Function:</u> <b>syncase:eval</b><i> expression</i></dt>
<dd><p><code>macro:eval</code> returns the value of <var>expression</var> in the current
top level environment.  <var>expression</var> can contain macro definitions.
Side effects of <var>expression</var> will affect the top level
environment.
</p></dd></dl>

<dl>
<dt><a name="index-macro_003aload-3"></a><u>Procedure:</u> <b>macro:load</b><i> filename</i></dt>
<dt><a name="index-syncase_003aload"></a><u>Procedure:</u> <b>syncase:load</b><i> filename</i></dt>
<dd><p><var>filename</var> should be a string.  If filename names an existing file,
the <code>macro:load</code> procedure reads Scheme source code expressions and
definitions from the file and evaluates them sequentially.  These
source code expressions and definitions may contain macro definitions.
The <code>macro:load</code> procedure does not affect the values returned by
<code>current-input-port</code> and <code>current-output-port</code>.
</p></dd></dl>

<p>This is version 2.1 of <code>syntax-case</code>, the low-level macro facility
proposed and implemented by Robert Hieb and R. Kent Dybvig.
</p>
<p>This version is further adapted by Harald Hanche-Olsen
&lt;hanche @ imf.unit.no&gt; to make it compatible with, and easily usable
with, SLIB.  Mainly, these adaptations consisted of:
</p>
<ul>
<li>
Removing white space from &lsquo;<tt>expand.pp</tt>&rsquo; to save space in the
distribution.  This file is not meant for human readers anyway&hellip;

</li><li>
Removed a couple of Chez scheme dependencies.

</li><li>
Renamed global variables used to minimize the possibility of name
conflicts.

</li><li>
Adding an SLIB-specific initialization file.

</li><li>
Removing a couple extra files, most notably the documentation (but see
below).
</li></ul>

<p>If you wish, you can see exactly what changes were done by reading the
shell script in the file &lsquo;<tt>syncase.sh</tt>&rsquo;.
</p>
<p>The two PostScript files were omitted in order to not burden the SLIB
distribution with them.  If you do intend to use <code>syntax-case</code>,
however, you should get these files and print them out on a PostScript
printer.  They are available with the original <code>syntax-case</code>
distribution by anonymous FTP in
&lsquo;<tt>cs.indiana.edu:/pub/scheme/syntax-case</tt>&rsquo;.
</p>
<p>In order to use syntax-case from an interactive top level, execute:
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(require 'syntax-case)
<a name="index-syntax_002dcase-2"></a>(require 'repl)
<a name="index-repl"></a>(repl:top-level macro:eval)
</pre></td></tr></table>
<p>See the section Repl (see section <a href="slib_7.html#Repl">Repl</a>) for more information.
</p>
<p>To check operation of syntax-case get
&lsquo;<tt>cs.indiana.edu:/pub/scheme/syntax-case</tt>&rsquo;, and type
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">(require 'syntax-case)
<a name="index-syntax_002dcase-3"></a><a name="index-syncase_003asanity_002dcheck"></a>(syncase:sanity-check)
</pre></td></tr></table>

<p>Beware that <code>syntax-case</code> takes a long time to load &ndash; about 20s on
a SPARCstation SLC (with SCM) and about 90s on a Macintosh SE/30 (with
Gambit).
</p>
<hr size="6">
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<h3 class="subsection">3.6.1 Notes</h3>

<p>All R4RS syntactic forms are defined, including <code>delay</code>.  Along
with <code>delay</code> are simple definitions for <code>make-promise</code> (into
which <code>delay</code> expressions expand) and <code>force</code>.
</p>
<p><code>syntax-rules</code> and <code>with-syntax</code> (described in <cite>TR356</cite>)
are defined.
</p>
<p><code>syntax-case</code> is actually defined as a macro that expands into
calls to the procedure <code>syntax-dispatch</code> and the core form
<code>syntax-lambda</code>; do not redefine these names.
</p>
<p>Several other top-level bindings not documented in TR356 are created:
</p><ul>
<li> the &ldquo;hooks&rdquo; in &lsquo;<tt>hooks.ss</tt>&rsquo;
</li><li> the <code>build-</code> procedures in &lsquo;<tt>output.ss</tt>&rsquo;
</li><li> <code>expand-syntax</code> (the expander)
</li></ul>

<p>The syntax of define has been extended to allow <code>(define <var>id</var>)</code>,
which assigns <var>id</var> to some unspecified value.
</p>
<p>We have attempted to maintain R4RS compatibility where possible.  The
incompatibilities should be confined to &lsquo;<tt>hooks.ss</tt>&rsquo;.  Please let us
know if there is some incompatibility that is not flagged as such.
</p>
<p>Send bug reports, comments, suggestions, and questions to Kent Dybvig
(dyb @ iuvax.cs.indiana.edu).
</p>

<hr size="6">
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<a name="Define_002dStructure-1"></a>
<h2 class="section">3.7 Define-Structure</h2>

<p><code>(require 'structure)</code>
</p>
<p>Included with the <code>syntax-case</code> files was &lsquo;<tt>structure.scm</tt>&rsquo;
which defines a macro <code>define-structure</code>.  Here is its
documentation from Gambit 4.0:
</p>
<dl>
<dt><a name="index-define_002dstructure"></a><u>special form:</u> <b>define-structure</b><i> <var>name</var> <var>field</var>&hellip;</i></dt>
<dd>
<p>Record data types similar to Pascal records and C <code>struct</code>
types can be defined using the <code>define-structure</code> special form.
The identifier <var>name</var> specifies the name of the new data type.  The
structure name is followed by <var>k</var> identifiers naming each field of
the record.  The <code>define-structure</code> expands into a set of definitions
of the following procedures:
</p>
<ul>
<li>
&lsquo;<tt>make-</tt><var>name</var>&rsquo; &ndash; A <var>k</var> argument procedure which constructs
a new record from the value of its <var>k</var> fields.

</li><li>
&lsquo;<var>name</var><tt>?</tt>&rsquo; &ndash; A procedure which tests if its single argument
is of the given record type.

</li><li>
&lsquo;<var>name</var><tt>-</tt><var>field</var>&rsquo; &ndash; For each field, a procedure taking
as its single argument a value of the given record type and returning
the content of the corresponding field of the record.

</li><li>
&lsquo;<var>name</var><tt>-</tt><var>field</var><tt>-set!</tt>&rsquo; &ndash; For each field, a two
argument procedure taking as its first argument a value of the given
record type.  The second argument gets assigned to the corresponding
field of the record and the void object is returned.

</li></ul>

<p>Gambit record data types have a printed representation that includes
the name of the type and the name and value of each field.
</p>
<p>For example:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">&gt; <b>(define-structure point x y color)</b>
&gt; <b>(define p (make-point 3 5 'red))</b>
&gt; <b>p</b>
#&lt;point #3 x: 3 y: 5 color: red&gt;
&gt; <b>(point-x p)</b>
3
&gt; <b>(point-color p)</b>
red
&gt; <b>(point-color-set! p 'black)</b>
&gt; <b>p</b>
#&lt;point #3 x: 3 y: 5 color: black&gt;
</pre></td></tr></table>

</dd></dl>


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<a name="Define_002dRecord_002dType-1"></a>
<h2 class="section">3.8 Define-Record-Type</h2>

<p><code>(require 'define-record-type)</code> or <code>(require 'srfi-9)</code>
<a name="index-srfi_002d9"></a>
<a name="index-define_002drecord_002dtype"></a>
</p>
<p><a href="http://srfi.schemers.org/srfi-9/srfi-9.html">http://srfi.schemers.org/srfi-9/srfi-9.html</a>
</p>
<dl>
<dt><a name="index-define_002drecord_002dtype-1"></a><u>Special Form:</u> <b>define-record-type</b><i> &lt;type-name&gt; (&lt;constructor-name&gt; &lt;field-tag&gt; ...) &lt;predicate-name&gt; &lt;field-spec&gt; ...</i></dt>
<dd>
<p>Where
</p><table><tr><td>&nbsp;</td><td><pre class="lisp">&lt;field-spec&gt; &equiv; (&lt;field-tag&gt; &lt;accessor-name&gt;)
             &equiv; (&lt;field-tag&gt; &lt;accessor-name&gt; &lt;modifier-name&gt;)

</pre></td></tr></table>

<p><code>define-record-type</code> is a syntax wrapper for the SLIB
<code>record</code> module.
</p></dd></dl>


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<a name="Fluid_002dLet-1"></a>
<h2 class="section">3.9 Fluid-Let</h2>

<p><code>(require 'fluid-let)</code>
<a name="index-fluid_002dlet"></a>
</p>
<dl>
<dt><a name="index-fluid_002dlet-1"></a><u>Syntax:</u> <b>fluid-let</b><i> <code>(<var>bindings</var> &hellip;)</code> <var>forms</var>&hellip;</i></dt>
</dl>
<table><tr><td>&nbsp;</td><td><pre class="lisp">(fluid-let ((<var>variable</var> <var>init</var>) &hellip;)
   <var>expression</var> <var>expression</var> &hellip;)
</pre></td></tr></table>

<p>The <var>init</var>s are evaluated in the current environment (in some
unspecified order), the current values of the <var>variable</var>s are saved,
the results are assigned to the <var>variable</var>s, the <var>expression</var>s
are evaluated sequentially in the current environment, the
<var>variable</var>s are restored to their original values, and the value of
the last <var>expression</var> is returned.
</p>
<p>The syntax of this special form is similar to that of <code>let</code>, but
<code>fluid-let</code> temporarily rebinds existing <var>variable</var>s.  Unlike
<code>let</code>, <code>fluid-let</code> creates no new bindings; instead it
<em>assigns</em> the values of each <var>init</var> to the binding (determined
by the rules of lexical scoping) of its corresponding
<var>variable</var>.
</p>

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<a name="Binding-to-multiple-values-1"></a>
<h2 class="section">3.10 Binding to multiple values</h2>

<p><code>(require 'receive)</code> or <code>(require 'srfi-8)</code>
<a name="index-srfi_002d8"></a>
<a name="index-receive"></a>
</p>
<dl>
<dt><a name="index-receive-1"></a><u>Special Form:</u> <b>receive</b><i> formals expression body &hellip;</i></dt>
<dd>
<p><a href="http://srfi.schemers.org/srfi-8/srfi-8.html">http://srfi.schemers.org/srfi-8/srfi-8.html</a>
</p></dd></dl>

<p><code>(require 'let-values)</code> or <code>(require 'srfi-11)</code>
<a name="index-srfi_002d11"></a>
<a name="index-let_002dvalues"></a>
</p>
<dl>
<dt><a name="index-let_002dvalues-1"></a><u>Special Form:</u> <b>let-values</b><i> ((formals expression) ...) body &hellip;</i></dt>
<dt><a name="index-let_002dvalues_002a"></a><u>Special Form:</u> <b>let-values*</b><i> ((formals expression) ...) body &hellip;</i></dt>
<dd>
<p><a href="http://srfi.schemers.org/srfi-11/srfi-11.html">http://srfi.schemers.org/srfi-11/srfi-11.html</a>
</p></dd></dl>


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<a name="Guarded-LET_002a-special-form-1"></a>
<h2 class="section">3.11 Guarded LET* special form</h2>

<p><code>(require 'and-let*)</code> or <code>(require 'srfi-2)</code>
<a name="index-srfi_002d2"></a>
<a name="index-and_002dlet_002a"></a>
</p>
<dl>
<dt><a name="index-and_002dlet_002a-1"></a><u>Macro:</u> <b>and-let*</b><i> claws body &hellip;</i></dt>
<dd>
<p><a href="http://srfi.schemers.org/srfi-2/srfi-2.html">http://srfi.schemers.org/srfi-2/srfi-2.html</a>
</p></dd></dl>


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<a name="Guarded-COND-Clause-1"></a>
<h2 class="section">3.12 Guarded COND Clause</h2>

<p><code>(require 'guarded-cond-clause)</code> or <code>(require 'srfi-61)</code>
<a name="index-srfi_002d61"></a>
<a name="index-guarded_002dcond_002dclause"></a>
</p>
<p><a href="http://srfi.schemers.org/srfi-61/srfi-61.html">http://srfi.schemers.org/srfi-61/srfi-61.html</a>
</p>
<dl>
<dt><a name="index-cond"></a><u>library syntax:</u> <b>cond</b><i>  &lt;clause1&gt; &lt;clause2&gt; &hellip;</i></dt>
<dd>
<p><em>Syntax:</em>
Each <span class="roman">&lt;clause&gt;</span> should be of the form
</p>
<table><tr><td>&nbsp;</td><td><pre class="format"><tt>(<span class="roman">&lt;test&gt;</span> <span class="roman">&lt;expression1&gt;</span> &hellip;)
</tt>
</pre></td></tr></table>

<p>where <span class="roman">&lt;test&gt;</span> is any expression.  Alternatively, a <span class="roman">&lt;clause&gt;</span> may be
of the form
</p>
<table><tr><td>&nbsp;</td><td><pre class="format"><tt>(<span class="roman">&lt;test&gt;</span> =&gt; <span class="roman">&lt;expression&gt;</span>)
</tt>
</pre></td></tr></table>

<p>The <span class="roman">&lt;clause&gt;</span> production in the formal syntax of Scheme as
written by R5RS in section 7.1.3 is extended with a new option:
<a name="index-_003d_003e"></a>
</p>
<table><tr><td>&nbsp;</td><td><pre class="format"><tt><span class="roman">&lt;clause&gt;</span> =&gt; (<span class="roman">&lt;generator&gt;</span> <span class="roman">&lt;guard&gt;</span> =&gt; <span class="roman">&lt;receiver&gt;</span>)
</tt>
</pre></td></tr></table>

<p>where <span class="roman">&lt;generator&gt;</span>, <span class="roman">&lt;guard&gt;</span>, &amp; <span class="roman">&lt;receiver&gt;</span> are all
<span class="roman">&lt;expression&gt;</span>s.
</p>
<blockquote><p>Clauses of this form have the following semantics: <span class="roman">&lt;generator&gt;</span> is
evaluated.  It may return arbitrarily many values.  <span class="roman">&lt;Guard&gt;</span> is
applied to an argument list containing the values in order that
<span class="roman">&lt;generator&gt;</span> returned.  If <span class="roman">&lt;guard&gt;</span> returns a true value for
that argument list, <span class="roman">&lt;receiver&gt;</span> is applied with an equivalent
argument list.  If <span class="roman">&lt;guard&gt;</span> returns a false value, however, the
clause is abandoned and the next one is tried.
</p></blockquote>

<p>The last <span class="roman">&lt;clause&gt;</span> may be
an &ldquo;else clause,&rdquo; which has the form
</p>
<table><tr><td>&nbsp;</td><td><pre class="format"><tt>(else <span class="roman">&lt;expression1&gt;</span> <span class="roman">&lt;expression2&gt;</span> &hellip;)<span class="roman">.</span>
</tt>
</pre></td></tr></table>
</dd></dl>

<p>This <code>port-&gt;char-list</code> procedure accepts an input port and
returns a list of all the characters it produces until the end.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">(define (port-&gt;char-list port)
  (cond ((read-char port) char?
         =&gt; (lambda (c) (cons c (port-&gt;char-list port))))
        (else '())))

(call-with-input-string &quot;foo&quot; port-&gt;char-list)  ==&gt;  (#\f #\o #\o)
</pre></td></tr></table>


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<a name="Yasos-1"></a>
<h2 class="section">3.13 Yasos</h2>


<p><code>(require 'oop)</code> or <code>(require 'yasos)</code>
<a name="index-oop"></a>
<a name="index-yasos"></a>
</p>
<p>&lsquo;Yet Another Scheme Object System&rsquo; is a simple object system for Scheme
based on the paper by Norman Adams and Jonathan Rees: <cite>Object
Oriented Programming in Scheme</cite>, Proceedings of the 1988 ACM Conference
on LISP and Functional Programming, July 1988 [ACM #552880].
</p>
<p>Another reference is:
</p>
<p>Ken Dickey.
&lt;A HREF=&quot;ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/swob.txt&quot;&gt;
Scheming with Objects
&lt;/A&gt;
<cite>AI Expert</cite> Volume 7, Number 10 (October 1992), pp. 24-33.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Yasos-terms">3.13.1 Terms</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                 Definitions and disclaimer.
</td></tr>
<tr><td align="left" valign="top"><a href="#Yasos-interface">3.13.2 Interface</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">             The Yasos macros and procedures.
</td></tr>
<tr><td align="left" valign="top"><a href="#Setters">3.13.3 Setters</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                     Dylan-like setters in Yasos.
</td></tr>
<tr><td align="left" valign="top"><a href="#Yasos-examples">3.13.4 Examples</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">              Usage of Yasos and setters.
</td></tr>
</table>

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<a name="Terms"></a>
<h3 class="subsection">3.13.1 Terms</h3>

<dl compact="compact">
<dt> <em>Object</em></dt>
<dd><p>Any Scheme data object.
</p>
</dd>
<dt> <em>Instance</em></dt>
<dd><p>An instance of the OO system; an <em>object</em>.
</p>
</dd>
<dt> <em>Operation</em></dt>
<dd><p>A <var>method</var>.
</p></dd>
</dl>

<dl compact="compact">
<dt> <em>Notes:</em></dt>
<dd><p>The object system supports multiple inheritance.  An instance can
inherit from 0 or more ancestors.  In the case of multiple inherited
operations with the same identity, the operation used is that from the
first ancestor which contains it (in the ancestor <code>let</code>).  An
operation may be applied to any Scheme data object&mdash;not just instances.
As code which creates instances is just code, there are no <em>classes</em>
and no meta-<var>anything</var>.  Method dispatch is by a procedure call a la
CLOS rather than by <code>send</code> syntax a la Smalltalk.
</p>
</dd>
<dt> <em>Disclaimer:</em></dt>
<dd><p>There are a number of optimizations which can be made.  This
implementation is expository (although performance should be quite
reasonable).  See the L&amp;FP paper for some suggestions.
</p></dd>
</dl>


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<a name="Interface"></a>
<h3 class="subsection">3.13.2 Interface</h3>

<dl>
<dt><a name="index-define_002doperation"></a><u>Syntax:</u> <b>define-operation</b><i> <code>(</code>opname self arg &hellip;<code>)</code> <var>default-body</var></i></dt>
<dd><p>Defines a default behavior for data objects which don&rsquo;t handle the
operation <var>opname</var>.  The default behavior (for an empty
<var>default-body</var>) is to generate an error.
</p></dd></dl>

<dl>
<dt><a name="index-define_002dpredicate"></a><u>Syntax:</u> <b>define-predicate</b><i> opname?</i></dt>
<dd><p>Defines a predicate <var>opname?</var>, usually used for determining the
<em>type</em> of an object, such that <code>(<var>opname?</var> <var>object</var>)</code>
returns <code>#t</code> if <var>object</var> has an operation <var>opname?</var> and
<code>#f</code> otherwise.
</p></dd></dl>

<dl>
<dt><a name="index-object"></a><u>Syntax:</u> <b>object</b><i> <code>((<var>name</var> <var>self</var> <var>arg</var> &hellip;) <var>body</var>)</code> &hellip;</i></dt>
<dd><p>Returns an object (an instance of the object system) with operations.
Invoking <code>(<var>name</var> <var>object</var> <var>arg</var> &hellip;</code> executes the
<var>body</var> of the <var>object</var> with <var>self</var> bound to <var>object</var> and
with argument(s) <var>arg</var>&hellip;.
</p></dd></dl>

<dl>
<dt><a name="index-object_002dwith_002dancestors"></a><u>Syntax:</u> <b>object-with-ancestors</b><i> <code>((</code>ancestor1 init1<code>)</code> &hellip;<code>)</code> operation &hellip;</i></dt>
<dd><p>A <code>let</code>-like form of <code>object</code> for multiple inheritance.  It
returns an object inheriting the behaviour of <var>ancestor1</var> etc.  An
operation will be invoked in an ancestor if the object itself does not
provide such a method.  In the case of multiple inherited operations
with the same identity, the operation used is the one found in the first
ancestor in the ancestor list.
</p></dd></dl>

<dl>
<dt><a name="index-operate_002das"></a><u>Syntax:</u> <b>operate-as</b><i> component operation self arg &hellip;</i></dt>
<dd><p>Used in an operation definition (of <var>self</var>) to invoke the
<var>operation</var> in an ancestor <var>component</var> but maintain the object&rsquo;s
identity.  Also known as &ldquo;send-to-super&rdquo;.
</p></dd></dl>

<dl>
<dt><a name="index-print"></a><u>Procedure:</u> <b>print</b><i> obj port</i></dt>
<dd><p>A default <code>print</code> operation is provided which is just <code>(format
<var>port</var> <var>obj</var>)</code> (see section <a href="slib_4.html#Format">Format (version 3.1)</a>) for non-instances and prints
<var>obj</var> preceded by &lsquo;<samp>#&lt;INSTANCE&gt;</samp>&rsquo; for instances.
</p></dd></dl>

<dl>
<dt><a name="index-size"></a><u>Function:</u> <b>size</b><i> obj</i></dt>
<dd><p>The default method returns the number of elements in <var>obj</var> if it is
a vector, string or list, <code>2</code> for a pair, <code>1</code> for a character
and by default id an error otherwise.  Objects such as collections
(see section <a href="slib_7.html#Collections">Collections</a>) may override the default in an obvious way.
</p></dd></dl>


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<a name="Setters-1"></a>
<h3 class="subsection">3.13.3 Setters</h3>

<p><em>Setters</em> implement <em>generalized locations</em> for objects
associated with some sort of mutable state.  A <em>getter</em> operation
retrieves a value from a generalized location and the corresponding
setter operation stores a value into the location.  Only the getter is
named &ndash; the setter is specified by a procedure call as below.  (Dylan
uses special syntax.)  Typically, but not necessarily, getters are
access operations to extract values from Yasos objects (see section <a href="#Yasos">Yasos</a>).
Several setters are predefined, corresponding to getters <code>car</code>,
<code>cdr</code>, <code>string-ref</code> and <code>vector-ref</code> e.g., <code>(setter
car)</code> is equivalent to <code>set-car!</code>.
</p>
<p>This implementation of setters is similar to that in Dylan(TM)
(<cite>Dylan: An object-oriented dynamic language</cite>, Apple Computer
Eastern Research and Technology).  Common LISP provides similar
facilities through <code>setf</code>.
</p>
<dl>
<dt><a name="index-setter"></a><u>Function:</u> <b>setter</b><i> getter</i></dt>
<dd><p>Returns the setter for the procedure <var>getter</var>.  E.g., since
<code>string-ref</code> is the getter corresponding to a setter which is
actually <code>string-set!</code>:
</p><table><tr><td>&nbsp;</td><td><pre class="example">(define foo &quot;foo&quot;)
((setter string-ref) foo 0 #\F) ; set element 0 of foo
foo &rArr; &quot;Foo&quot;
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-set"></a><u>Syntax:</u> <b>set</b><i> place new-value</i></dt>
<dd><p>If <var>place</var> is a variable name, <code>set</code> is equivalent to
<code>set!</code>.  Otherwise, <var>place</var> must have the form of a procedure
call, where the procedure name refers to a getter and the call indicates
an accessible generalized location, i.e., the call would return a value.
The return value of <code>set</code> is usually unspecified unless used with a
setter whose definition guarantees to return a useful value.
</p><table><tr><td>&nbsp;</td><td><pre class="example">(set (string-ref foo 2) #\O)  ; generalized location with getter
foo &rArr; &quot;FoO&quot;
(set foo &quot;foo&quot;)               ; like set!
foo &rArr; &quot;foo&quot;
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-add_002dsetter"></a><u>Procedure:</u> <b>add-setter</b><i> getter setter</i></dt>
<dd><p>Add procedures <var>getter</var> and <var>setter</var> to the (inaccessible) list
of valid setter/getter pairs.  <var>setter</var> implements the store
operation corresponding to the <var>getter</var> access operation for the
relevant state.  The return value is unspecified.
</p></dd></dl>

<dl>
<dt><a name="index-remove_002dsetter_002dfor"></a><u>Procedure:</u> <b>remove-setter-for</b><i> getter</i></dt>
<dd><p>Removes the setter corresponding to the specified <var>getter</var> from the
list of valid setters.  The return value is unspecified.
</p></dd></dl>

<dl>
<dt><a name="index-define_002daccess_002doperation"></a><u>Syntax:</u> <b>define-access-operation</b><i> getter-name</i></dt>
<dd><p>Shorthand for a Yasos <code>define-operation</code> defining an operation
<var>getter-name</var> that objects may support to return the value of some
mutable state.  The default operation is to signal an error.  The return
value is unspecified.
</p></dd></dl>


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<a name="Examples-1"></a>
<h3 class="subsection">3.13.4 Examples</h3>

<table><tr><td>&nbsp;</td><td><pre class="lisp">;;; These definitions for PRINT and SIZE are
;;; already supplied by
(require 'yasos)

(define-operation (print obj port)
  (format port
          (if (instance? obj) &quot;#&lt;instance&gt;&quot; &quot;~s&quot;)
          obj))

(define-operation (size obj)
  (cond
   ((vector? obj) (vector-length obj))
   ((list?   obj) (length obj))
   ((pair?   obj) 2)
   ((string? obj) (string-length obj))
   ((char?   obj) 1)
   (else
    (slib:error &quot;Operation not supported: size&quot; obj))))

(define-predicate cell?)
(define-operation (fetch obj))
(define-operation (store! obj newValue))

(define (make-cell value)
  (object
   ((cell? self) #t)
   ((fetch self) value)
   ((store! self newValue)
    (set! value newValue)
    newValue)
   ((size self) 1)
   ((print self port)
    (format port &quot;#&lt;Cell: ~s&gt;&quot; (fetch self)))))

(define-operation (discard obj value)
  (format #t &quot;Discarding ~s~%&quot; value))

(define (make-filtered-cell value filter)
  (object-with-ancestors
   ((cell (make-cell value)))
   ((store! self newValue)
   (if (filter newValue)
       (store! cell newValue)
       (discard self newValue)))))

(define-predicate array?)
(define-operation (array-ref array index))
(define-operation (array-set! array index value))

(define (make-array num-slots)
  (let ((anArray (make-vector num-slots)))
    (object
     ((array? self) #t)
     ((size self) num-slots)
     ((array-ref self index)
      (vector-ref  anArray index))
     ((array-set! self index newValue)
      (vector-set! anArray index newValue))
     ((print self port)
      (format port &quot;#&lt;Array ~s&gt;&quot; (size self))))))

(define-operation (position obj))
(define-operation (discarded-value obj))

(define (make-cell-with-history value filter size)
  (let ((pos 0) (most-recent-discard #f))
    (object-with-ancestors
     ((cell (make-filtered-call value filter))
      (sequence (make-array size)))
     ((array? self) #f)
     ((position self) pos)
     ((store! self newValue)
      (operate-as cell store! self newValue)
      (array-set! self pos newValue)
      (set! pos (+ pos 1)))
     ((discard self value)
      (set! most-recent-discard value))
     ((discarded-value self) most-recent-discard)
     ((print self port)
      (format port &quot;#&lt;Cell-with-history ~s&gt;&quot;
              (fetch self))))))

(define-access-operation fetch)
(add-setter fetch store!)
(define foo (make-cell 1))
(print foo #f)
&rArr; &quot;#&lt;Cell: 1&gt;&quot;
(set (fetch foo) 2)
&rArr;
(print foo #f)
&rArr; &quot;#&lt;Cell: 2&gt;&quot;
(fetch foo)
&rArr; 2
</pre></td></tr></table>


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