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<H2>Issue SYMBOL-MACROLET-TYPE-DECLARATION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss339.htm">SYMBOL-MACROLET-TYPE-DECLARATION</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/s_symbol.htm#symbol-macrolet"><B>SYMBOL-MACROLET</B></A>, <A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A><P>
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Related issues: <A HREF="iss337.htm">SYMBOL-MACROLET-DECLARE</A><P>
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<A HREF="iss342.htm">SYNTACTIC-ENVIRONMENT-ACCESS</A><P>
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<B>Category:</B> CLARIFICATON<P>
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<B>Edit history:</B> v1, 13 Feb 1991, Sandra Loosemore<P>
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v2, 11 Mar 1991, Sandra Loosemore (new proposal PROBABLY)<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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Proposal <A HREF="iss337.htm">SYMBOL-MACROLET-DECLARE:ALLOW</A> says that a type declaration of<P>
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a symbol defined as a symbol-macro "is equivalent to wrapping a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A><P>
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expression around the expansion of that symbol". There is disagreement<P>
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about whether "equivalence" was intended to mean "semantic equivalence"<P>
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or "literal equivalence". In other words, must (or might) the value <P>
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returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> include a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> form if there<P>
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are type declarations that apply to the symbol-macro being expanded?<P>
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<P>
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There are four proposals, YES, NO, MAYBE, and PROBABLY.<P>
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<P>
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<B>Proposal (SYMBOL-MACROLET-TYPE-DECLARATION:YES):<P>
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</B><P>
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Clarify that the equivalence is literal. If a type declaration<P>
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is provided for a name that is bound as a symbol-macro, that type<P>
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information must appear in the expansion of the symbol-macro <P>
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returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> as a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> form wrapped around <P>
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the expansion provided in its definition.<P>
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<P>
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Rationale for proposal YES:<P>
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<P>
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If the macroexpansion doesn't include the type information, it can<P>
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be lost completely.<P>
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<P>
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<P>
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<B>Proposal (SYMBOL-MACROLET-TYPE-DECLARATION:NO):<P>
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</B><P>
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Clarify that the equivalence is only in effect. The macroexpansion <P>
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of a symbol-macro returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> is exactly<P>
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the expansion provided in its definition.<P>
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<P>
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Rationale for proposal NO:<P>
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<P>
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As long as we permit implementations to discard declarations as a <P>
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matter of policy, it's unreasonable type declarations on symbol<P>
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macros to be collected.<P>
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<P>
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Type declarations about symbol-macros need not be lost during<P>
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macroexpansion, since this information is returned by the<P>
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VARIABLE-INFORMATION function. A code walker could wrap a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A><P>
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form around the expansion returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> itself, if<P>
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it wished to do so.<P>
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<P>
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Maybe some programs depend on the expansion of a symbol-macro being<P>
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exactly the same as what was specified in its definition.<P>
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<P>
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<P>
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<B>Proposal (SYMBOL-MACROLET-TYPE-DECLARATION:MAYBE):<P>
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</B><P>
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Clarify that the equivalence may be either literal or only in effect.<P>
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The macroexpansion of a symbol-macro returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or<P>
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<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> might or might not include a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> form wrapped around<P>
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the the expansion provided in its definition.<P>
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<P>
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Rationale for proposal MAYBE:<P>
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<P>
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Some implementations might want to do things this way. Other<P>
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implementations might not.<P>
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<P>
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<P>
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<B>Proposal (SYMBOL-MACROLET-TYPE-DECLARATION:PROBABLY):<P>
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</B><P>
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Clarify that the equivalence is literal. If there is type information<P>
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available in the environment in which the expansion occurs, then that<P>
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type information must appear in the expansion of the symbol-macro <P>
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returned by <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> as a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> form wrapped around <P>
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the expansion provided in its definition.<P>
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<P>
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Rationale for proposal PROBABLY:<P>
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<P>
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If the macroexpansion doesn't include the type information, it can<P>
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be lost completely. However, this proposal makes no requirement that<P>
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type declarations actually be collected (but it does imply that<P>
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type declarations added by an explicit use of AUGMENT-ENVIRONMENT<P>
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will be reflected in the expansions).<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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I don't know.<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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In an implementation that discards type declarations, probably quite<P>
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a bit of work would be involved to support proposal YES. However it<P>
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may be that the implementors are doing this work anyway to support<P>
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the other <A HREF="iss342.htm">syntactic-environment-access</A> proposals.<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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Hard to say. Probably there are few programs that depend on any<P>
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particular behavior here.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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Continuing confusion about how to write a code walker.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Most of the overhead is in keeping track of type declarations.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The cost of non-adoption is avoided.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Seems to depend on who you talk to.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Kim Barrett supports proposal PROBABLY.<P>
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<P>
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Kent Pitman says:<P>
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...unless we're going to force declarations to be enforced, it <P>
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isn't reasonable to make this requirement on macroexpansion.<P>
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<P>
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Loosemore supports proposal NO, not only because of this problem,<P>
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but because it just seems unnecessarily complicated to force <P>
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<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> to deal with declarations when there are other mechanisms <P>
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for retrieving this information available to code walkers.<P>
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<P>
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</PRE>
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<HR>
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