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<H2>Issue MACROEXPAND-RETURN-VALUE Writeup</H2>
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<PRE><B>Forum:</B> Cleanup<P>
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<B>Issue:</B> <A HREF="iss235.htm">MACROEXPAND-RETURN-VALUE</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A>, <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A><P>
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(CLtL p151)<P>
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COMPILER-MACROEXPAND, COMPILER-MACROEXPAND-1<P>
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(issue <A REL=DEFINITION HREF="../Body/m_define.htm#define-compiler-macro"><B>DEFINE-COMPILER-MACRO</B></A>)<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit History:</B> V1, 29 Oct 1989, Sandra Loosemore<P>
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<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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CLtL says that the second value returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> and<P>
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<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> is T if the argument form is a macro call. Presumably<P>
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this is also the case for COMPILER-MACROEXPAND and<P>
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COMPILER-MACROEXPAND-1, since these functions are intended to be "just<P>
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like" their counterparts. Specifying a return value of T is<P>
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inconsistent with other predicates in the language, where a return<P>
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value of "true" is normally specified instead.<P>
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<P>
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<P>
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<B>Proposal (MACROEXPAND-RETURN-VALUE:TRUE):<P>
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</B><P>
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Change the specification so that the second return value from<P>
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<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A>, <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A>, COMPILER-MACROEXPAND, and<P>
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COMPILER-MACROEXPAND-1 is a <A REL=DEFINITION HREF="../Body/t_ban.htm#boolean"><B>boolean</B></A> instead of one of the symbols T or<P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>. In other words, if the form represents a macro call (or a<P>
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compiler macro call, as appropriate), the second return value is true.<P>
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<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This is more consistent with other predicates in the language.<P>
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<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Presumably all conforming implementations now return T.<P>
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<P>
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<P>
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<B>Cost to implementors:<P>
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</B><P>
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None, except possibly for the cost of changing documentation.<P>
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<P>
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<P>
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<B>Cost to users:<P>
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</B><P>
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It seems unlikely that user programs would explicitly test for the<P>
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return value being T, or otherwise depend on this.<P>
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<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Increased consistency in the language design.<P>
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<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Loosemore notes:<P>
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In some implementations, there is a slight performance advantage in<P>
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allowing the return values from predicates to be any non-NIL value<P>
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instead of the symbol T. Some functions such as <A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> are required<P>
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to return a particular truth value that conveys extra information back<P>
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to the caller, but there doesn't appear to be similar motivation for<P>
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making an exception to the general rule in this situation.<P>
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</PRE>
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