199 lines
9.4 KiB
HTML
199 lines
9.4 KiB
HTML
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<title>
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Revised^5 Report on the Algorithmic Language Scheme
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</title>
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<link rel="stylesheet" type="text/css" href="r5rs-Z-C.css" title=default>
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<meta name=robots content="noindex,follow">
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</head>
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<body>
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<p><div class=navigation>[Go to <span><a href="r5rs.html">first</a>, <a href="r5rs-Z-H-7.html">previous</a></span><span>, <a href="r5rs-Z-H-9.html">next</a></span> page<span>; </span><span><a href="r5rs-Z-H-2.html#%_toc_start">contents</a></span><span><span>; </span><a href="r5rs-Z-H-15.html#%_index_start">index</a></span>]</div><p>
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<a name="%_chap_5"></a>
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<h1 class=chapter>
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<div class=chapterheading><a href="r5rs-Z-H-2.html#%_toc_%_chap_5">Chapter 5</a></div><p>
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<a href="r5rs-Z-H-2.html#%_toc_%_chap_5">Program structure</a></h1><p>
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<p>
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<a name="%_sec_5.1"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_5.1">5.1 Programs</a></h2><p>
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A Scheme program consists of a sequence of expressions, definitions,
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and syntax definitions.
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Expressions are described in chapter <a href="r5rs-Z-H-7.html#%_chap_4">4</a>;
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definitions and syntax definitions are the subject of the rest of the
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present chapter.<p>
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Programs are typically stored in files or entered interactively to a
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running Scheme system, although other paradigms are possible;
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questions of user interface lie outside the scope of this report.
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(Indeed, Scheme would still be useful as a notation for expressing
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computational methods even in the absence of a mechanical
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implementation.)<p>
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Definitions and syntax definitions occurring at the top level of a program
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can be interpreted
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declaratively.
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They cause bindings to be created in the top level
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environment or modify the value of existing top-level bindings.
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Expressions occurring at the top level of a program are
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interpreted imperatively; they are executed in order when the program is
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invoked or loaded, and typically perform some kind of initialization.<p>
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At the top level of a program <tt>(begin <form<sub>1</sub>> <tt>...</tt>)</tt> is
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equivalent to the sequence of expressions, definitions, and syntax definitions
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that form the body of the <tt>begin</tt>.<p>
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<p>
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<a name="%_sec_5.2"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_5.2">5.2 Definitions</a></h2><p>
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<p>
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Definitions are valid in some, but not all, contexts where expressions
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are allowed. They are valid only at the top level of a <program>
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and at the beginning of a <body>.
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<a name="%_idx_188"></a><p>
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A definition should have one of the following forms:<a name="%_idx_190"></a><p>
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<p><ul><p>
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<li><tt>(define <variable> <expression>)</tt><p>
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<li><tt>(define (<variable> <formals>) <body>)</tt><p>
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<Formals> should be either a
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sequence of zero or more variables, or a sequence of one or more
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variables followed by a space-delimited period and another variable (as
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in a lambda expression). This form is equivalent to
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<tt><p>(define <variable><br>
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(lambda (<formals>) <body>)).<p></tt><p>
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<li><tt>(define (<variable> . <formal>) <body>)</tt><p>
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<Formal> should be a single
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variable. This form is equivalent to
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<tt><p>(define <variable><br>
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(lambda <formal> <body>)).<p></tt><p>
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</ul><p><p>
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<a name="%_sec_5.2.1"></a>
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<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_5.2.1">5.2.1 Top level definitions</a></h3><p>
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At the top level of a program, a definition
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<tt><p>(define <variable> <expression>)<p></tt>
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has essentially the same effect as the assignment expression
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<tt><p>(<tt>set!</tt> <variable> <expression>)<p></tt>
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if <variable> is bound. If <variable> is not bound,
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however, then the definition will bind <variable> to a new
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location before performing the assignment, whereas it would be an error
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to perform a <tt>set!</tt> on an unbound<a name="%_idx_192"></a> variable.<p>
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<tt><p>(define add3<br>
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(lambda (x) (+ x 3)))<br>
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(add3 3) ===> 6<br>
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(define first car)<br>
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(first '(1 2)) ===> 1<p></tt><p>
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Some implementations of Scheme use an initial environment in
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which all possible variables are bound to locations, most of
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which contain undefined values. Top level definitions in
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such an implementation are truly equivalent to assignments.<p>
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<p>
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<a name="%_sec_5.2.2"></a>
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<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_5.2.2">5.2.2 Internal definitions</a></h3><p>
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<p>
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Definitions may occur at the
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beginning of a <body> (that is, the body of a <tt>lambda</tt>,
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<tt>let</tt>, <tt>let*</tt>, <tt>letrec</tt>, <tt>let-syntax</tt>, or <tt>letrec-syntax</tt>
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expression or that of a definition of an appropriate form).
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Such definitions are known as <em>internal definitions</em> <a name="%_idx_194"></a> as opposed to the top level definitions described above.
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The variable defined by an internal definition is local to the
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<body>. That is, <variable> is bound rather than assigned,
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and the region of the binding is the entire <body>. For example,<p>
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<tt><p>(let ((x 5))<br>
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(define foo (lambda (y) (bar x y)))<br>
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(define bar (lambda (a b) (+ (* a b) a)))<br>
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(foo (+ x 3))) ===> 45<p></tt><p>
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A <body> containing internal definitions can always be converted
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into a completely equivalent <tt>letrec</tt> expression. For example, the
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<tt>let</tt> expression in the above example is equivalent to<p>
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<tt><p>(let ((x 5))<br>
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(letrec ((foo (lambda (y) (bar x y)))<br>
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(bar (lambda (a b) (+ (* a b) a))))<br>
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(foo (+ x 3))))<p></tt><p>
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Just as for the equivalent <tt>letrec</tt> expression, it must be
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possible to evaluate each <expression> of every internal
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definition in a <body> without assigning or referring to
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the value of any <variable> being defined.<p>
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Wherever an internal definition may occur
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<tt>(begin <definition<sub>1</sub>> <tt>...</tt>)</tt>
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is equivalent to the sequence of definitions
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that form the body of the <tt>begin</tt>.<p>
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<a name="%_sec_5.3"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_5.3">5.3 Syntax definitions</a></h2><p>
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Syntax definitions are valid only at the top level of a <program>.
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<a name="%_idx_196"></a>
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They have the following form:<a name="%_idx_198"></a><p>
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<tt>(define-syntax <keyword> <transformer spec>)</tt><p>
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<Keyword> is an identifier, and
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the <transformer spec> should be an instance of <tt>syntax-rules</tt>.
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The top-level syntactic environment is extended by binding the
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<keyword> to the specified transformer.<p>
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There is no <tt>define-syntax</tt> analogue of internal definitions.<p>
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Although macros may expand into definitions and syntax definitions in
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any context that permits them, it is an error for a definition or syntax
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definition to shadow a syntactic keyword whose meaning is needed to
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determine whether some form in the group of forms that contains the
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shadowing definition is in fact a definition, or, for internal definitions,
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is needed to determine the boundary between the group and the expressions
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that follow the group. For example, the following are errors:<p>
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<tt><p>(define define 3)<br>
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<br>
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(begin (define begin list))<br>
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<br>
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(let-syntax<br>
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((foo (syntax-rules ()<br>
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((foo (proc args ...) body ...)<br>
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(define proc<br>
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(lambda (args ...)<br>
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body ...))))))<br>
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(let ((x 3))<br>
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(foo (plus x y) (+ x y))<br>
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(define foo x)<br>
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(plus foo x)))<br>
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<p></tt><p>
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<p>
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<p><div class=navigation>[Go to <span><a href="r5rs.html">first</a>, <a href="r5rs-Z-H-7.html">previous</a></span><span>, <a href="r5rs-Z-H-9.html">next</a></span> page<span>; </span><span><a href="r5rs-Z-H-2.html#%_toc_start">contents</a></span><span><span>; </span><a href="r5rs-Z-H-15.html#%_index_start">index</a></span>]</div><p>
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</body>
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