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5.7 KiB
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94 lines
5.7 KiB
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<title>CLiki: Issue DEFINE-COMPILER-MACRO-DECLINE</title>
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<span class="hidden">CLiki - Issue DEFINE-COMPILER-MACRO-DECLINE</span>
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<div id="content"><div id="content-area"><div id="article-title">Issue DEFINE-COMPILER-MACRO-DECLINE</div><div id="article"><a href="Issue.html" class="category">Issue</a>: DEFINE-COMPILER-MACRO-DECLINE<p>Forum: Editorial<p>References: <a href="https://www.cliki.net/site/HyperSpec/Body/mac_define-compiler-macro.html" class="hyperspec">DEFINE-COMPILER-MACRO</a><p>Category: CLARIFICATION/CHANGE<p>Edit history: 2004-06-16, Version 1 by Christophe Rhodes<p>Status: For <a href="CLiki.html" class="internal">CLiki</a> consideration<p><dl>
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<dt>Problem Description:
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</dt>
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<dd>
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The glossary entry for <a href="http://www.lispworks.com/reference/HyperSpec/Body/26_glo_c.htm#compiler_macro_function">compiler macro function</a> is inconsistent with the entries in the main body of the standard: the glossary states that returning <a href="https://www.cliki.net/site/HyperSpec/Body/any_nil.html" class="hyperspec">nil</a> from the compiler macro function indicates that the original form should not be expanded, whereas <a href="https://www.cliki.net/site/HyperSpec/Body/mac_define-compiler-macro.html" class="hyperspec">define-compiler-macro</a> states that this is achieved by returning the original form (from the <a href="https://www.cliki.net/site/HyperSpec/Body/sec_3-4-4.html" class="hyperspec">&whole</a> argument).<p></dd>
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<dt>Proposal (DEFINE-COMPILER-MACRO-DECLINE:ORIGINAL-FORM-ONLY):
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</dt>
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<dd>
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Specify that the glossary entry was in error, and that to decline expansion the original form should be returned from the compiler macro function. Specify that a return value of <a href="https://www.cliki.net/site/HyperSpec/Body/any_nil.html" class="hyperspec">nil</a> indicates that the form should be expanded by the compiler, if it expands the compiler macro at all, to <a href="https://www.cliki.net/site/HyperSpec/Body/any_nil.html" class="hyperspec">nil</a>.<p></dd>
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<dt>Test case:
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</dt>
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<dd><p></dd>
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<dt>Rationale:
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</dt>
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<dd>
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The glossary entry was clearly in error, and could potentially cause confusion.<p></dd>
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<dt>Current practice:
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</dt>
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<dd>
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<a href="SBCL.html" class="internal">SBCL</a> 0.8.11 (and earlier versions), <a href="CMUCL.html" class="internal">CMUCL</a> 18e and <a href="CLISP.html" class="internal">CLISP</a> 2.33 behave as in proposal DEFINE-COMPILER-MACRO-DECLINE:ORIGINAL-FORM-ONLY.<p></dd>
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<dt>Cost to Implementors:
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</dt>
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<dd>
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Minimal.<p></dd>
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<dt>Cost to Users:
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</dt>
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<dd>
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None.<p></dd>
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<dt>Cost of Non-Adoption:
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</dt>
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<dd>
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Continued confusion over the required semantics of a compiler macro function.<p></dd>
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<dt>Benefits:
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</dt>
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<dd>
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Enhanced ability to write compiler macros portably.<p></dd>
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<dt>Aesthetics:
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</dt>
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<dd>
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Returning the original form to decline is clearly superior to returning <a href="https://www.cliki.net/site/HyperSpec/Body/any_nil.html" class="hyperspec">nil</a>, as it avoids any possible confusion over <a href="https://www.cliki.net/site/HyperSpec/Body/any_nil.html" class="hyperspec">nil</a> as expansion.<p></dd>
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<dt>Discussion:
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</dt>
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<dd><p></dd>
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