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<TITLE>CLHS: Issue THE-VALUES Writeup</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss346_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss347.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss348_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue THE-VALUES Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss347.htm">THE-VALUES</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> special form, <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> type specifier<P>
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Related issues: <A HREF="iss346.htm">THE-AMBIGUITY</A><P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> v1, 24 Feb 1991, Sandra Loosemore<P>
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<B>Status:</B> X3J13 passed v1:RETURN-NUMBER-RECEIVED on vote of 10-1, Mar-91<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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Are the following forms valid, and if so how many values are returned <P>
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in each case?<P>
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<P>
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#1: (truncate 7 3)<P>
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#2: (the integer (truncate 7 3))<P>
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#3: (the (values integer) (truncate 7 3))<P>
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#4: (the (values integer integer) (truncate 7 3))<P>
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#5: (the (values integer integer symbol) (truncate 7 3))<P>
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<P>
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There are two proposals, RETURN-NUMBER-RECEIVED and STRICT-MATCHING.<P>
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<P>
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<B>Proposal (THE-VALUES:RETURN-NUMBER-RECEIVED):<P>
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</B><P>
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(1) Clarify that the <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> special form returns the exact values as <P>
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its subform.<P>
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<P>
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(2) Clarify that it is not an error if the subform returns more values <P>
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than what is specified by the type specifier, provided that the<P>
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values for which types are declared are indeed of those types.<P>
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(In particular, <A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> <A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>type</B></A> specifier is <A REL=DEFINITION HREF="../Body/a_not.htm#not"><B>not</B></A> a <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> <A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>type</B></A> <P>
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specifier, multiple <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>values</B></A> may still be returned.)<P>
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<P>
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(3) Clarify that it is not an error if the subform returns fewer values<P>
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than what is specified by the type specifier. Missing values are<P>
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treated as <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> for the purposes of checking their types.<P>
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<P>
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Rationale for proposal RETURN-NUMBER-RECEIVED:<P>
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<P>
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Point by point,<P>
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<P>
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(1) <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> is supposed to act purely as a declaration. It should not<P>
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change the number of values that are returned by its subform.<P>
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<P>
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(2) This is consistent with the way extra values are simply ignored<P>
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by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> and friends.<P>
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<P>
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(3) This is consistent with the way missing values are treated as <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> and friends.<P>
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<P>
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<P>
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<B>Proposal (THE-VALUES:STRICT-MATCHING):<P>
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</B><P>
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(1) Clarify that the <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> special form returns the exact values as <P>
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its subform.<P>
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<P>
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(2) Clarify that it is an error if the number and syntax of values <P>
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returned by the subform do not match the given type specifier<P>
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exactly. In particular, if the type specifier is not a <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A><P>
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type specifier then only a single value may be returned.<P>
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<P>
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Rationale for proposal STRICT-MATCHING:<P>
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<P>
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Point by point,<P>
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<P>
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(1) <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> is supposed to act purely as a declaration. It should not<P>
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change the number of values that are returned by its subform.<P>
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<P>
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(2) This is consistent with the description of the <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> type <P>
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specifier as specifying the arguments of a function which could<P>
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capture the values via <A REL=DEFINITION HREF="../Body/s_multip.htm#multiple-value-call"><B>MULTIPLE-VALUE-CALL</B></A>. Since argument<P>
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mismatches are clearly "is an error" situations, this should be<P>
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too. This is also apparently closer to current practice than<P>
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proposal RETURN-NUMBER-RECEIVED.<P>
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<P>
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<B>Examples:<P>
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</B><P>
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Under proposal RETURN-NUMBER-RECEIVED, all of the examples given in <P>
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the problem description section are valid and return the values 2 and 1.<P>
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<P>
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Under proposal STRICT-MATCHING, tests 2, 3, and 5 are in error. Tests<P>
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1 and 4 are valid and return the values 2 and 1.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Lucid CL version 4.0 signals an error in cases 3 and 5 (at least in the<P>
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interpreter). It returns the values 2 and 1 in case 2.<P>
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<P>
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Allegro CL version 3.1 signals an error in cases 2, 3, and 5.<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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Probably small. Neither proposal requires checking to be performed in<P>
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any situation. However under proposal RETURN-NUMBER-RECEIVED, some <P>
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implementations will have to be changed to not do some checks they <P>
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currently do.<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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Users may find the stricter error checking permitted by proposal <P>
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STRICT-MATCHING helpful. However, if implementations that currently<P>
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do not check these errors begin to do so, some programs that currently<P>
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work in those implementations to stop working.<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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Confusion about what the <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> type specifier really means.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Probably small, but proposal STRICT-MATCHING might permit compilers<P>
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to be more aggressive about some kinds of type and multiple-value<P>
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optimizations.<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. In the best of all possible<P>
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worlds, <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> and friends would have ordinary lambda-list<P>
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syntax and follow the same rules as for ordinary lambda-binding, so there <P>
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wouldn't be the an inconsistency with <A REL=DEFINITION HREF="../Body/s_multip.htm#multiple-value-call"><B>MULTIPLE-VALUE-CALL</B></A> to have to<P>
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resolve one way or the other.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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<P>
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This issue was discussed on the common-lisp mailing list some time ago,<P>
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but never written up. Here's the last message from that discussion:<P>
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<P>
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Date: Thu, 20 Apr 1989 17:53 EDT<P>
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From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM><P>
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<P>
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Date: Thu, 20 Apr 89 16:59:15 EDT<P>
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From: Guy Steele <gls@Think.COM><P>
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...<P>
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PROPOSAL:<P>
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<P>
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What it boils down to, is that <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> should check only as many types<P>
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as requested (and pass back only as many).<P>
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<P>
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No, this is not cool. <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> is supposed to act purely as a declaration,<P>
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but you are changing it to <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> it to pass on only as many values<P>
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as the type specifer indicates. This could change the semantics of<P>
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a suitably devious program.<P>
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<P>
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Better to say that it checks as many types as requsted, but passes on<P>
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exactly the values it receives.<P>
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--Guy<P>
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<P>
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Even though I agree with your position, I think it's worth our writing up<P>
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a clarification issue to make sure we're all agreed and that it's 100% <P>
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clear in the ANSI spec.<P>
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-------<P>
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</PRE>
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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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