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<TITLE>CLHS: Issue FUNCTION-TYPE-KEY-NAME Writeup</TITLE>
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<H2>Issue FUNCTION-TYPE-KEY-NAME Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss177.htm">FUNCTION-TYPE-KEY-NAME</A><P>
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<B>References:</B> CLtL p.47-48, 61<P>
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<B>Category:</B> CLARIFICATION, CHANGE<P>
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<B>Edit history:</B> Version 1, 23-Nov-1987 Sandra Loosemore<P>
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Version 2, 15-Jan-1988 Sandra Loosemore <P>
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(from comments by Kent Pitman)<P>
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Version 3, 13-Feb-88 Masinter<P>
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Related issues: <A HREF="iss178.htm">FUNCTION-TYPE-REST-LIST-ELEMENT</A>, <P>
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<A HREF="iss208.htm">KEYWORD-ARGUMENT-NAME-PACKAGE</A><P>
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FUNCTION-ARGUMENT-TYPE-SEMANTICS<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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The <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier list is provided to allow declaration of function<P>
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argument types and return value types. This type specifier uses a syntax<P>
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similar to the usual <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list syntax to specify which types go with which<P>
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<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list variables. However, there is a problem with <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> variables<P>
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because CLtL does not specify how the types specified in the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A><P>
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declaration are matched up to either the actual arguments passed to the<P>
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function, or the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> variables in the function definition (since the ordering<P>
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of keyword arguments is arbitrary).<P>
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<P>
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<B>Proposal (FUNCTION-TYPE-KEY-NAME:SPECIFY-KEYWORD):<P>
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</B><P>
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(1) Specify that the <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> parameters in a <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list<P>
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should be supplied as lists of the form (<keyword> <type>). The <keyword> must<P>
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be a valid keyword-name symbol as must be supplied in the actual arguments of a<P>
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call. (This is usually a symbol in the keyword package, but, as per<P>
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<A HREF="iss208.htm">KEYWORD-ARGUMENT-NAME-PACKAGE</A>, not necessarily so.) <P>
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<P>
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(2) Allow <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> to appear in a <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list. <P>
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<P>
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The interpretation of such declarations is that, when <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> is given in a<P>
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<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list, it is safe to assume that the &KEYs given<P>
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are exhaustive unless <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> is present. <P>
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<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> is an indication that other keyword arguments may actually be<P>
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supplied and, if supplied, may be used. <P>
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<P>
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<B>Example:<P>
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</B><P>
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The type of the function <A REL=DEFINITION HREF="../Body/f_mk_lis.htm#make-list"><B>MAKE-LIST</B></A> could be declared as:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> <A REL=DEFINITION HREF="../Body/f_mk_lis.htm#make-list"><B>MAKE-LIST</B></A> ((<A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A> 0) <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> (:INITIAL-ELEMENT T)) <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A>)<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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(1) This specifies a direct correspondence between the argument type and its<P>
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matching keyword. All of the information is in one place, and the user does not<P>
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have to remember (or even know) the order in which <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> arguments appear in the<P>
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actual function definition.<P>
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<P>
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(2) This is probably an oversight in the original specification.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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Many Common Lisp implementations currently ignore <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type declarations.<P>
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The situation regarding type specifications for keyword arguments is so<P>
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ambiguous that few users attempt to use them.<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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Implementations that ignore the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier or keyword arguments in<P>
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a <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier may continue to do so. This proposal should not<P>
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involve massive amounts of code to be rewritten.<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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Because the current situation is so ambiguous, <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifiers and<P>
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particularly the specification of keyword argument types are not widely used.<P>
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However, since this is an incompatible change, it would be nice if<P>
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implementations check for, and warn about, old-style usage.<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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If nothing is done, the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type specifier will continue to be of limited<P>
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use for its intended purpose.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Adopting the proposal will clear up an area of confusion in the language design.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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The syntax is fairly obvious and is analogous to the (<keyword> <variable>)<P>
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<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list syntax.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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The exact semantics of function declarations and the types of arguments is<P>
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still under discussion, as are several other issues dealing with declarations.<P>
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However, this issue seemed separable.<P>
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<P>
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</PRE>
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