266 lines
12 KiB
HTML
266 lines
12 KiB
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<title>
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Revised^6 Report on the Algorithmic Language Scheme
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</title>
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<link rel="stylesheet" type="text/css" href="r6rs-Z-S.css" title=default>
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<meta name=robots content="index,follow">
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</head>
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<body>
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<div id=slidecontent>
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<div align=right class=navigation>[Go to <span><a href="r6rs.html">first</a>, <a href="r6rs-Z-H-12.html">previous</a></span><span>, <a href="r6rs-Z-H-14.html">next</a></span> page<span>; </span><span><a href="r6rs-Z-H-2.html#node_toc_start">contents</a></span><span><span>; </span><a href="r6rs-Z-H-21.html#node_index_start">index</a></span>]</div>
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<p></p>
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<a name="node_chap_10"></a>
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<h1 class=chapter>
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<div class=chapterheading><a href="r6rs-Z-H-2.html#node_toc_node_chap_10">Chapter 10</a></div><br>
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<a href="r6rs-Z-H-2.html#node_toc_node_chap_10">Expansion process</a></h1>
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<p></p>
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<p>
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Macro uses (see section <a href="r6rs-Z-H-12.html#node_sec_9.2">9.2</a>) are expanded into <i>core
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forms</i><a name="node_idx_322"></a>at the start of evaluation (before compilation or interpretation)
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by a syntax <em>expander</em>.
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The set of core forms is implementation-dependent, as is the
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representation of these forms in the expander's output.
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If the expander encounters a syntactic abstraction, it invokes
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the associated transformer to expand the syntactic abstraction, then
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repeats the expansion process for the form returned by the transformer.
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If the expander encounters a core form, it recursively
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processes its subforms that are in expression or definition context,
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if any, and reconstructs the form from the
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expanded subforms.
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Information about identifier bindings is maintained during expansion
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to enforce lexical scoping for variables and keywords.</p>
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<p>
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To handle definitions, the expander processes the initial
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forms in a <body> (see section <a href="r6rs-Z-H-14.html#node_sec_11.3">11.3</a>) or
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<library body> (see section <a href="r6rs-Z-H-10.html#node_sec_7.1">7.1</a>)
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from left to
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right. How the expander processes each form encountered
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depends upon the kind of form.</p>
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<p>
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</p>
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<dl><dt></dt><dd>
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</dd><dt><b>macro use</b></dt><dd>
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The expander invokes the associated transformer to transform the macro
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use, then recursively performs whichever of these actions are appropriate
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for the resulting form.<p>
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</p>
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</dd><dt><b><tt>define-syntax</tt> form</b></dt><dd>
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The expander expands and evaluates the right-hand-side expression and binds the
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keyword to the resulting transformer.<p>
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</p>
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</dd><dt><b><tt>define</tt> form</b></dt><dd>
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The expander records the fact that the defined identifier is a variable but defers
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expansion of the right-hand-side expression until after all of the
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definitions have been processed.<p>
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</p>
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</dd><dt><b><tt>begin</tt> form</b></dt><dd>
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The expander splices the subforms into the list of body forms it is
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processing. (See section <a href="r6rs-Z-H-14.html#node_sec_11.4.7">11.4.7</a>.)<p>
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</p>
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</dd><dt><b><tt>let-syntax</tt> or <tt>letrec-syntax</tt> form</b></dt><dd>
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The expander splices the inner body forms into the list of (outer) body forms it is
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processing, arranging for the keywords bound by the <tt>let-syntax</tt>
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and <tt>letrec-syntax</tt> to be visible only in the inner body forms.<p>
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</p>
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</dd><dt><b>expression, i.e., nondefinition</b></dt><dd>
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The expander completes the expansion of the deferred right-hand-side expressions
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and the current and remaining expressions in the body, and then
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creates the equivalent of a <tt>letrec*</tt> form from the defined variables,
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expanded right-hand-side expressions, and expanded body expressions.
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</dd></dl><p></p>
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<p>
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For the right-hand side of the definition of a variable, expansion is
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deferred until after all of the definitions have been seen. Consequently,
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each keyword and variable reference within the right-hand side
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resolves to the local binding, if any.</p>
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<p>
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A definition in the sequence of forms must not define any identifier whose
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binding is used to determine the meaning of the undeferred portions of the
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definition or any definition that precedes it in the sequence of forms.
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For example, the bodies of the following expressions violate this
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restriction.</p>
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<p>
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</p>
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<tt>(let ()
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<p class=nopadding> (define define 17)</p>
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<p class=nopadding> (list define))</p>
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<p class=nopadding></p>
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<p class=nopadding>(let-syntax ([def0 (syntax-rules ()</p>
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<p class=nopadding> [(_ x) (define x 0)])])</p>
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<p class=nopadding> (let ([z 3])</p>
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<p class=nopadding> (def0 z)</p>
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<p class=nopadding> (define def0 list)</p>
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<p class=nopadding> (list z)))</p>
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<p class=nopadding></p>
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<p class=nopadding>(let ()</p>
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<p class=nopadding> (define-syntax foo</p>
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<p class=nopadding> (lambda (e)</p>
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<p class=nopadding> (+ 1 2)))</p>
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<p class=nopadding> (define + 2)</p>
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<p class=nopadding> (foo))</p>
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<p></tt></p>
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<p>
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The following do not violate the restriction.</p>
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<p>
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</p>
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<tt>(let ([x 5])
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<p class=nopadding> (define lambda list)</p>
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<p class=nopadding> (lambda x x)) <span style="margin-left: 2em">‌</span>⇒ (5 5)</p>
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<p class=nopadding></p>
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<p class=nopadding>(let-syntax ([def0 (syntax-rules ()</p>
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<p class=nopadding> [(_ x) (define x 0)])])</p>
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<p class=nopadding> (let ([z 3])</p>
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<p class=nopadding> (define def0 list)</p>
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<p class=nopadding> (def0 z)</p>
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<p class=nopadding> (list z))) <span style="margin-left: 2em">‌</span>⇒ (3)</p>
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<p class=nopadding></p>
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<p class=nopadding>(let ()</p>
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<p class=nopadding> (define-syntax foo</p>
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<p class=nopadding> (lambda (e)</p>
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<p class=nopadding> (let ([+ -]) (+ 1 2))))</p>
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<p class=nopadding> (define + 2)</p>
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<p class=nopadding> (foo)) <span style="margin-left: 2em">‌</span>⇒ -1</p>
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<p></tt></p>
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<p>
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The implementation should treat a violation of the restriction as a
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syntax violation.</p>
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<p>
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</p>
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<p>
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Note that this algorithm does not directly reprocess any form.
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It requires a single left-to-right pass over the definitions followed by a
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single pass (in any order) over the body expressions and deferred
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right-hand sides.</p>
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<p>
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Example:</p>
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<p>
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</p>
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<tt>(lambda (x)
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<p class=nopadding> (define-syntax defun</p>
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<p class=nopadding> (syntax-rules ()</p>
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<p class=nopadding> [(_ x a e) (define x (lambda a e))]))</p>
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<p class=nopadding> (defun even? (n) (or (= n 0) (odd? (- n 1))))</p>
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<p class=nopadding> (define-syntax odd?</p>
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<p class=nopadding> (syntax-rules () [(_ n) (not (even? n))]))</p>
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<p class=nopadding> (odd? (if (odd? x) (* x x) x)))</p>
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<p></tt></p>
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<p>
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In the example, the definition of <tt>defun</tt> is encountered first, and the keyword
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<tt>defun</tt> is associated with the transformer resulting from
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the expansion and evaluation of the corresponding right-hand side.
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A use of <tt>defun</tt> is encountered next and expands into a
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<tt>define</tt> form.
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Expansion of the right-hand side of this define form is deferred.
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The definition of <tt>odd?</tt> is next and results in the association
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of the keyword <tt>odd?</tt> with the transformer resulting from
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expanding and evaluating the corresponding right-hand side.
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A use of <tt>odd?</tt> appears next and is expanded; the resulting
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call to <tt>not</tt> is recognized as an expression
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because <tt>not</tt> is bound as a variable.
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At this point, the expander completes the expansion of the current
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expression (the call to <tt>not</tt>) and the deferred right-hand side of the
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<tt>even?</tt> definition;
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the uses of <tt>odd?</tt> appearing in these expressions are expanded
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using the transformer associated with the keyword <tt>odd?</tt>.
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The final output is the equivalent of</p>
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<p>
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</p>
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<tt>(lambda (x)
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<p class=nopadding> (letrec* ([even?</p>
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<p class=nopadding> (lambda (n)</p>
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<p class=nopadding> (or (= n 0)</p>
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<p class=nopadding> (not (even? (- n 1)))))])</p>
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<p class=nopadding> (not (even? (if (not (even? x)) (* x x) x)))))</p>
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<p></tt></p>
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<p>
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although the structure of the output is implementation-dependent.</p>
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<p>
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Because definitions and expressions can be interleaved in a
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<top-level body> (see chapter <a href="r6rs-Z-H-11.html#node_chap_8">8</a>),
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the expander's processing of a <top-level body> is somewhat
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more complicated.
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It behaves as described above for a <body> or
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<library body> with the following exceptions:
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When the expander finds a nondefinition,
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it defers its expansion and continues scanning for definitions.
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Once it reaches the end of the set of forms, it processes the
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deferred right-hand-side and body expressions, then
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generates the equivalent of a <tt>letrec*</tt> form from the defined variables,
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expanded right-hand-side expressions, and expanded body expressions.
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For each body expression <expression> that appears before a variable definition
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in the body, a dummy binding is created at the corresponding place within
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the set of <tt>letrec*</tt> bindings, with a fresh temporary variable on the
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left-hand side and the equivalent of <tt>(begin <expression>
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<unspecified>)</tt>,
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where <unspecified> is a side-effect-free expression returning
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an unspecified value,
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on the right-hand side, so that
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left-to-right evaluation order is preserved.
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The <tt>begin</tt> wrapper allows <expression> to evaluate to an
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arbitrary number of values.</p>
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<p>
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</p>
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<p></p>
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<div class=smallskip></div>
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<p style="margin-top: 0pt; margin-bottom: 0pt">
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<div align=right class=navigation>[Go to <span><a href="r6rs.html">first</a>, <a href="r6rs-Z-H-12.html">previous</a></span><span>, <a href="r6rs-Z-H-14.html">next</a></span> page<span>; </span><span><a href="r6rs-Z-H-2.html#node_toc_start">contents</a></span><span><span>; </span><a href="r6rs-Z-H-21.html#node_index_start">index</a></span>]</div>
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