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<title>CLiki: Issue DEFMACRO-VALUES</title>
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<span class="hidden">CLiki - Issue DEFMACRO-VALUES</span>
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<div id="content"><div id="content-area"><div id="article-title">Issue DEFMACRO-VALUES</div><div id="article"><a href="Issue.html" class="category">Issue</a>: DEFMACRO-VALUES<p>Forum:<p>References: <a href="https://www.cliki.net/site/HyperSpec/Body/mac_defmacro.html" class="hyperspec">DEFMACRO</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/speope_fletcm_scm_macrolet.html" class="hyperspec">MACROLET</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a>,
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<a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/acc_macro-function.html" class="hyperspec">MACRO-FUNCTION</a>, the Glossary definition for
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<a href="http://www.cliki.net/site/HyperSpec/Body/glo_v.html#value">value</a>.<p>Category:<p>Edit history:
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2005-09-11, Version 1 by Kalle Olavi Niemitalo<p>Status: For CLiki consideration<p><dl>
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<dt>Problem Description:
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</dt>
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<dd>According to the description of <a href="https://www.cliki.net/site/HyperSpec/Body/mac_defmacro.html" class="hyperspec">DEFMACRO</a>,
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"the value of the last <i>form</i> executed is returned as the
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expansion of the macro." The glossary definition of
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<a href="http://www.cliki.net/site/HyperSpec/Body/glo_v.html#value">value</a>
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(1. b.) indicates that this means the primary value; any further
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values are ignored. But does this truncation occur as part of the
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expansion function defined by DEFMACRO, or as part of the
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macroexpansion process? This matters if the user calls the expansion
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function directly, or if the user's <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> cares.<p> This also concerns <a href="https://www.cliki.net/site/HyperSpec/Body/speope_fletcm_scm_macrolet.html" class="hyperspec">MACROLET</a>, which uses "the same format used
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by <a href="https://www.cliki.net/site/HyperSpec/Body/mac_defmacro.html" class="hyperspec">defmacro</a>."<p></dd>
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<dt>Proposal (DEFMACRO-VALUES:PRIMARY):
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</dt>
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<dd>The expansion functions defined by DEFMACRO or MACROLET always return
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exactly one value, even if the body attempts to return multiple values
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(as the values of the last form, or by using <a href="https://www.cliki.net/site/HyperSpec/Body/speope_return-from.html" class="hyperspec">RETURN-FROM</a>).
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With SETF <a href="https://www.cliki.net/site/HyperSpec/Body/acc_macro-function.html" class="hyperspec">MACRO-FUNCTION</a>, the user can specify a
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macro function that returns multiple values; but only the primary value
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matters when the macro function is being called by <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND</a>,
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<a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>, or whatever the implementation uses to expand
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macros.<p> The function that is the value of <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> is not
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required to preserve multiple values returned by the macro function,
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and it might not even see all the values if another hook has
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interposed itself.<p></dd>
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<dt>Rationale (DEFMACRO-VALUES:PRIMARY):
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</dt>
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<dd>According to the description of <a href="https://www.cliki.net/site/HyperSpec/Body/mac_defmacro.html" class="hyperspec">DEFMACRO</a>,
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"the expansion function returns a
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<a href="http://www.cliki.net/site/HyperSpec/Body/glo_f.html#form">form</a>."
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Make it behave that way.<p></dd>
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<dt>Proposal (DEFMACRO-VALUES:ALL):
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</dt>
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<dd>The expansion functions defined by DEFMACRO or MACROLET return all the
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values of the body, but only the primary value matters when the expansion
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function is being called by <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>,
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or whatever the implementation uses to expand macros.<p> The function that is the value of <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> is not
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required to preserve multiple values returned by the macro function,
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and it might not even see all the values if another hook has
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interposed itself.<p></dd>
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<dt>Rationale (DEFMACRO-VALUES:ALL):
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</dt>
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<dd>This seems to be what the implementations do now, and is helpful if
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users want to use MACROLET as a general way to add information to the
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lexical environment.<p></dd>
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<dt>Proposal (DEFMACRO-VALUES:EXPLICITLY-VAGUE):
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</dt>
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<dd>It is unspecified whether the expansion functions defined by
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DEFMACRO or MACROLET return all the values of the body, or just the
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primary value. With SETF <a href="https://www.cliki.net/site/HyperSpec/Body/acc_macro-function.html" class="hyperspec">MACRO-FUNCTION</a>, the user can
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specify a macro function that returns multiple values. In any case,
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only the primary value matters when the expansion function is being
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called by <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>, or whatever
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the implementation uses to expand macros.<p> The function that is the value of <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> is not
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required to preserve multiple values returned by the macro function,
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and it might not even see all the values if another hook has
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interposed itself.<p></dd>
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<dt>Rationale (DEFMACRO-VALUES:EXPLICITLY-VAGUE):
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</dt>
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<dd>Clarify the spec without requiring implementations to change.<p></dd>
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<dt>Proposal (DEFMACRO-VALUES:THROUGHOUT):
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</dt>
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<dd>The expansion functions defined by DEFMACRO and MACROLET return all the
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values of the body, but only the primary value matters when the expansion
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function is being called by <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>,
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or whatever the implementation uses to expand macros.<p> The function that is the value of <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> will see
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all the values returned by the macro function, and must preserve
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them. Because it cannot normally know how the extra values will be
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used, it should pass them through unchanged.<p></dd>
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<dt>Rationale (DEFMACRO-VALUES:THROUGHOUT):
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</dt>
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<dd>If users define macros that return multiple values, then the
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implementation and user-defined hooks should propagate them as far as
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possible, in case some hook somewhere knows what to do with them.
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Admittedly, this is rather far-fetched: because <a href="https://www.cliki.net/site/HyperSpec/Body/fun_macroexpa_acroexpand-1.html" class="hyperspec">MACROEXPAND-1</a>
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will discard the extra values anyway, users are unlikely to call it
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on such macros, and then the value of <a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a> won't
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get called either.<p></dd>
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<dt>Test case:
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</dt>
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<dd></dd>
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</dl><pre>
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(defmacro foo ()
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(values 1 2 3))
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(multiple-value-list (funcall (macro-function 'foo) '(foo) nil))
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;; returns (1) for DEFMACRO-VALUES:PRIMARY,
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;; returns (1 2 3) for DEFMACRO-VALUES:ALL or DEFMACRO-VALUES:THROUGHOUT,
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;; returns either for DEFMACRO-VALUES:EXPLICITLY-VAGUE.
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(let ((*macroexpand-hook*
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#'(lambda (function form env)
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(let ((values (multiple-value-list (funcall function form env))))
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(format *trace-output* "~&Values:~{ ~S~}~%" values)
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(values-list values)))))
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(macroexpand-1 '(foo)))
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;; outputs "Values: 1" for DEFMACRO-VALUES:PRIMARY,
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;; outputs "Values: 1 2 3" for DEFMACRO-VALUES:THROUGHOUT,
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;; outputs either for DEFMACRO-VALUES:ALL or DEFMACRO-VALUES:EXPLICITLY-VAGUE,
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;; always returns 1 and T.
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</pre><p><dt>Rationale:
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</dt><dd><p></dd><dt>Current practice:
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</dt><dd>
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<a href="SBCL.html" class="internal">SBCL</a> 0.9.3.51, <a href="CLISP.html" class="internal">CLISP</a> 2.34, and Allegro CL 6.2 seem to
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implement DEFMACRO-VALUES:ALL.<p></dd><dt>Cost to Implementors:
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</dt><dd>DEFMACRO-VALUES:EXPLICITLY-VAGUE is intended not to require any
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changes in implementations.<p> It seems possible that all current implementations support
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DEFMACRO-VALUES:ALL, in which case that would not require changes
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either. Some implementations may currently support
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DEFMACRO-VALUES:PRIMARY instead. Switching from either one to the
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other would require some effort, and might make macros expand a
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little slower.<p> DEFMACRO-VALUES:THROUGHOUT would additionally require implementors to
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examine and correct all of the functions they place in
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<a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a>.<p></dd><dt>Cost to Users:<p></dt><dd>DEFMACRO-VALUES:EXPLICITLY-VAGUE might make users' programs less
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portable, if all current implementations support DEFMACRO-VALUES:ALL
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and the programs have come to rely on that and some future implementation
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chooses to behave like DEFMACRO-VALUES:PRIMARY instead.<p> If all current implementations support DEFMACRO-VALUES:ALL, then making
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this official would not hurt users, but we don't know that yet.<p> Requiring DEFMACRO-VALUES:PRIMARY would break programs that haven't been
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careful about portability.<p> DEFMACRO-VALUES:THROUGHOUT would additionally require users to
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examine and correct all of the functions they place in
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<a href="https://www.cliki.net/site/HyperSpec/Body/var_stmacroexpand-hookst.html" class="hyperspec">*MACROEXPAND-HOOK*</a>.<p></dd><dt>Cost of Non-Adoption:
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</dt><dd>Unclear spec. Users don't know what they can rely on, and may
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assume too much in programs intended to be portable.<p></dd><dt>Benefits:
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</dt><dd><p></dd><dt>Aesthetics:
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</dt><dd><p></dd><dt>Discussion:
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</dt><dd><p></dd></div></div>
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