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<tr><td align="center" bgcolor="#8381de"><div class="skribetitle"><strong><big><big><big>5. R5Rs<br/>Scheme Revised(5) Report on the Algorithmic Language Scheme -- Program structure</big></big></big></strong></div><center>
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<tr><td valign="top" align="left">5.1</td><td colspan="4" width="100%"><a href="r5rs-8.html#Programs">Programs</a></td></tr>
<tr><td valign="top" align="left">5.2</td><td colspan="4" width="100%"><a href="r5rs-8.html#Definitions">Definitions</a></td></tr>
<tr><td></td><td valign="top" align="left">5.2.1</td><td colspan="3" width="100%"><a href="r5rs-8.html#Top-level-definitions">Top level definitions</a></td></tr>
<tr><td></td><td valign="top" align="left">5.2.2</td><td colspan="3" width="100%"><a href="r5rs-8.html#Internal-definitions">Internal definitions</a></td></tr>
<tr><td valign="top" align="left">5.3</td><td colspan="4" width="100%"><a href="r5rs-8.html#Syntax-definitions">Syntax definitions</a></td></tr>
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<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-1.html#Summary">Summary</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-2.html#Introduction">Introduction</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-3.html#Table-of-contents">Table of contents</a></td></tr>
<tr><td valign="top" align="left">1</td><td colspan="4" width="100%"><a href="r5rs-4.html#Overview-of-Scheme">Overview of Scheme</a></td></tr>
<tr><td valign="top" align="left">2</td><td colspan="4" width="100%"><a href="r5rs-5.html#Lexical-conventions">Lexical conventions</a></td></tr>
<tr><td valign="top" align="left">3</td><td colspan="4" width="100%"><a href="r5rs-6.html#Basic-concepts">Basic concepts</a></td></tr>
<tr><td valign="top" align="left">4</td><td colspan="4" width="100%"><a href="r5rs-7.html#Expressions">Expressions</a></td></tr>
<tr><td valign="top" align="left">5</td><td colspan="4" width="100%"><a href="r5rs-8.html#Program-structure">Program structure</a></td></tr>
<tr><td valign="top" align="left">6</td><td colspan="4" width="100%"><a href="r5rs-9.html#Standard-procedures">Standard procedures</a></td></tr>
<tr><td valign="top" align="left">7</td><td colspan="4" width="100%"><a href="r5rs-10.html#Formal-syntax-and-semantics">Formal syntax and semantics</a></td></tr>
<tr><td valign="top" align="left">8</td><td colspan="4" width="100%"><a href="r5rs-11.html#Concepts">Concepts</a></td></tr>
<tr><td valign="top" align="left">9</td><td colspan="4" width="100%"><a href="r5rs-12.html#Variables-and-Procedures">Variables and Procedures</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-13.html#Notes">Notes</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-14.html#Additional-material">Additional material</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-15.html#Example">Example</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-16.html#Bibliography">Bibliography</a></td></tr>
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<td align="left" valign="top" class="skribe-body"><div class="skribe-body">
<a name="Program-structure" class="mark"></a>
<!-- Programs -->
<a name="Programs"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">5.1 Programs</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Programs" class="mark"></a>
A Scheme program consists of a sequence of expressions, definitions,
and syntax definitions.
Expressions are described in chapter <a href="r5rs-7.html#Expressions" class="inbound">Expressions</a>;
definitions and syntax definitions are the subject of the rest of the
present chapter.<br/><br/>Programs are typically stored in files or entered interactively to a
running Scheme system, although other paradigms are possible;
questions of user interface lie outside the scope of this report.
(Indeed, Scheme would still be useful as a notation for expressing
computational methods even in the absence of a mechanical
implementation.)<br/><br/>Definitions and syntax definitions occurring at the top level of a program
can be interpreted
declaratively.
They cause bindings to be created in the top level
environment or modify the value of existing top-level bindings.
Expressions occurring at the top level of a program are
interpreted imperatively; they are executed in order when the program is
invoked or loaded, and typically perform some kind of initialization.<br/><br/>At the top level of a program <tt id='tt2517'
>(begin <form1> ...,)</tt> is
equivalent to the sequence of expressions, definitions, and syntax definitions
that form the body of the <code id='code2518'
>begin</code>.
<a name="g2520" class="mark"></a>
</div><br>
<!-- Definitions -->
<a name="Definitions"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">5.2 Definitions</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Definitions" class="mark"></a>
Definitions are valid in some, but not all, contexts where expressions
are allowed. They are valid only at the top level of a <program>
and at the beginning of a <body>.<br/><br/><a name="g2524" class="mark"></a>
A definition should have one of the following forms:
<a name="g2527" class="mark"></a>
<ul class="itemize" id='itemize2542'
><li><tt id='tt2529'
>(define <variable> <expression>)</tt>
</li>
<li><tt id='tt2531'
>(define (<variable> <formals>) <body>)</tt>
<Formals> should be either a
sequence of zero or more variables, or a sequence of one or more
variables followed by a space-delimited period and another variable (as
in a lambda expression). This form is equivalent to
<center id='center2534'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ccccff"><pre class="prog" id='prog2532'
>(define <variable>
(lambda (<formals>) <body>)).
</pre>
</td></tr>
</tbody></table></center>
</li>
<li><tt id='tt2536'
>(define (<variable> .: <formal>) <body>)</tt>
<Formal> should be a single
variable. This form is equivalent to<br/><br/><center id='center2540'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ccccff"><pre class="prog" id='prog2538'
>(define <variable>
(lambda <formal> <body>)).
</pre>
</td></tr>
</tbody></table></center>
</li>
</ul>
<!-- Top level definitions -->
<a name="Top-level-definitions"></a>
<div class="subsection-atitle"><table width="100%"><tr><td bgcolor="#ffffff"><h3><font color="#8381de">5.2.1 Top level definitions</font>
</h3></td></tr></table>
</div><div class="subsection">
<a name="Top-level-definitions" class="mark"></a>
At the top level of a program, a definition<br/><br/><center id='center2546'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ccccff"><pre class="prog" id='prog2544'
>(define <variable> <expression>)
</pre>
</td></tr>
</tbody></table></center>
has essentially the same effect as the assignment expression<br/><br/><center id='center2550'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ccccff"><pre class="prog" id='prog2548'
>(set! <variable> <expression>)
</pre>
</td></tr>
</tbody></table></center>
if <variable> is bound. If <variable> is not bound,
however, then the definition will bind <variable> to a new
location before performing the assignment, whereas it would be an error
to perform a <samp id='samp2551'
>set!</samp> on an unbound variable.
<a name="g2553" class="mark"></a>
<center id='center2560'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog2558'
>(<font color="#6959cf"><strong id='bold7820'
>define</strong></font> <font color="#6959cf"><strong id='bold7822'
>add3</strong></font>
(<strong id='bold7824'
>lambda</strong> (x) (+ x 3)))
(add3 3) => 6
(<font color="#6959cf"><strong id='bold7825'
>define</strong></font> <font color="#6959cf"><strong id='bold7827'
>first</strong></font> car)
(first '(1 2)) => 1
</pre>
</td></tr>
</tbody></table></center>
Some implementations of Scheme use an initial environment in
which all possible variables are bound to locations, most of
which contain undefined values. Top level definitions in
such an implementation are truly equivalent to assignments.<br/><br/>
</div>
<!-- Internal definitions -->
<a name="Internal-definitions"></a>
<div class="subsection-atitle"><table width="100%"><tr><td bgcolor="#ffffff"><h3><font color="#8381de">5.2.2 Internal definitions</font>
</h3></td></tr></table>
</div><div class="subsection">
<a name="Internal-definitions" class="mark"></a>
Definitions may occur at the
beginning of a <body> (that is, the body of a <code id='code2562'
>lambda</code>,
<a name="g2564" class="mark"></a><code id='code2566'
>let</code>, <code id='code2567'
>let*</code>, <code id='code2568'
>letrec</code>, <code id='code2569'
>let-syntax</code>, or <code id='code2570'
>letrec-syntax</code>
<a name="g2572" class="mark"></a><a name="g2575" class="mark"></a><a name="g2578" class="mark"></a><a name="g2581" class="mark"></a><a name="g2584" class="mark"></a>expression or that of a definition of an appropriate form).
Such definitions are known as <em id='emph2586'
>internal definitions</em> as opposed to the top level definitions described above.
<a name="g2588" class="mark"></a>The variable defined by an internal definition is local to the
<body>. That is, <variable> is bound rather than assigned,
and the region of the binding is the entire <body>. For example,<br/><br/><center id='center2598'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog2596'
>(<strong id='bold7829'
>let</strong> ((x 5))
(<font color="#6959cf"><strong id='bold7830'
>define</strong></font> <font color="#6959cf"><strong id='bold7832'
>foo</strong></font> (<strong id='bold7834'
>lambda</strong> (y) (bar x y)))
(<font color="#6959cf"><strong id='bold7835'
>define</strong></font> <font color="#6959cf"><strong id='bold7837'
>bar</strong></font> (<strong id='bold7839'
>lambda</strong> (a b) (+ (* a b) a)))
(foo (+ x 3))) => 45
</pre>
</td></tr>
</tbody></table></center>
A <body> containing internal definitions can always be converted
into a completely equivalent <samp id='samp2599'
>letrec</samp> expression. For example, the
<samp id='samp2600'
>let</samp> expression in the above example is equivalent to<br/><br/><center id='center2608'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog2606'
>(<strong id='bold7840'
>let</strong> ((x 5))
(<strong id='bold7841'
>letrec</strong> ((foo (<strong id='bold7842'
>lambda</strong> (y) (bar x y)))
(bar (<strong id='bold7843'
>lambda</strong> (a b) (+ (* a b) a))))
(foo (+ x 3))))
</pre>
</td></tr>
</tbody></table></center>
Just as for the equivalent <samp id='samp2609'
>letrec</samp> expression, it must be
possible to evaluate each <expression> of every internal
definition in a <body> without assigning or referring to
the value of any <variable> being defined.<br/><br/>Wherever an internal definition may occur
<tt id='tt2611'
>(begin <definition1> ...,)</tt>
is equivalent to the sequence of definitions
that form the body of the <code id='code2612'
>begin</code>.
<a name="g2614" class="mark"></a>
</div>
</div><br>
<!-- Syntax definitions -->
<a name="Syntax-definitions"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">5.3 Syntax definitions</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Syntax-definitions" class="mark"></a><br/><br/>Syntax definitions are valid only at the top level of a <program>.<br/><br/><a name="g2619" class="mark"></a>They have the following form:
<a name="g2622" class="mark"></a>
<tt id='tt2624'
>(define-syntax <keyword> <transformer spec>)</tt><br/><br/><Keyword> is an identifier, and
the <transformer spec> should be an instance of <code id='code2626'
>syntax-rules</code>.
<a name="g2628" class="mark"></a>The top-level syntactic environment is extended by binding the
<keyword> to the specified transformer.<br/><br/>There is no <samp id='samp2631'
>define-syntax</samp> analogue of internal definitions.<br/><br/>
Although macros may expand into definitions and syntax definitions in
any context that permits them, it is an error for a definition or syntax
definition to shadow a syntactic keyword whose meaning is needed to
determine whether some form in the group of forms that contains the
shadowing definition is in fact a definition, or, for internal definitions,
is needed to determine the boundary between the group and the expressions
that follow the group. For example, the following are errors:<br/><br/><center id='center2646'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog2644'
>(<font color="#6959cf"><strong id='bold7844'
>define</strong></font> <font color="#6959cf"><strong id='bold7846'
>define</strong></font> 3)<br/><br/>(<strong id='bold7848'
>begin</strong> (<font color="#6959cf"><strong id='bold7849'
>define</strong></font> <font color="#6959cf"><strong id='bold7851'
>begin</strong></font> list))<br/><br/>(let-syntax
((foo (<strong id='bold7853'
>syntax-rules</strong> ()
((foo (proc args ...) body ...)
(<font color="#6959cf"><strong id='bold7854'
>define</strong></font> <font color="#6959cf"><strong id='bold7856'
>proc</strong></font>
(<strong id='bold7858'
>lambda</strong> (args ...)
body ...))))))
(<strong id='bold7859'
>let</strong> ((x 3))
(foo (plus x y) (+ x y))
(<font color="#6959cf"><strong id='bold7860'
>define</strong></font> <font color="#6959cf"><strong id='bold7862'
>foo</strong></font> x)
(plus foo x)))
</pre>
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