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<H2>Issue MULTIPLE-VALUES-LIMIT-ON-VARIABLES Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss246.htm">MULTIPLE-VALUES-LIMIT-ON-VARIABLES</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/m_mult_2.htm#multiple-value-setq"><B>MULTIPLE-VALUE-SETQ</B></A>, <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/m_nth_va.htm#nth-value"><B>NTH-VALUE</B></A><P>
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Related issues: none<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> v1, 13 Feb 1991, Sandra Loosemore<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 constant <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A> is defined in CLtL as the upper<P>
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exclusive bound on the number of values that may be returned from<P>
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a function. Does this limit also apply to the number of variables<P>
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that may be bound or assigned by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> or <P>
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MULTIPLE-VALUE-SETQ? Or are the excess variables simply given values<P>
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of NIL?<P>
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<P>
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There are two proposals, UNDEFINED and <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>.<P>
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<P>
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<B>Proposal (MULTIPLE-VALUES-LIMIT-ON-VARIABLES:UNDEFINED):<P>
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</B><P>
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Clarify that <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A> applies to the number of variables<P>
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that may be bound or assigned by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> or <P>
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<A REL=DEFINITION HREF="../Body/m_mult_2.htm#multiple-value-setq"><B>MULTIPLE-VALUE-SETQ</B></A> and the index argument to <A REL=DEFINITION HREF="../Body/m_nth_va.htm#nth-value"><B>NTH-VALUE</B></A>, as well <P>
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as to the number values that can actually be returned.<P>
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<P>
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Rationale:<P>
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<P>
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It's conceivable that these forms for accessing multiple values <P>
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could be implemented in such a way that they break if you try to<P>
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access values beyond <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A>.<P>
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<P>
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<P>
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<B>Proposal (MULTIPLE-VALUES-LIMIT-ON-VARIABLES:NIL):<P>
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</B><P>
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Clarify that <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A> applies only to the number of values<P>
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that can actually be returned, and not to the number of variables that<P>
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may be bound or assigned by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A> or <A REL=DEFINITION HREF="../Body/m_mult_2.htm#multiple-value-setq"><B>MULTIPLE-VALUE-SETQ</B></A><P>
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or the index argument to <A REL=DEFINITION HREF="../Body/m_nth_va.htm#nth-value"><B>NTH-VALUE</B></A>. As usual, <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> if there are not<P>
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enough values actually returned.<P>
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<P>
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Rationale:<P>
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<P>
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There are ways of implementing these forms for accessing multiple<P>
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values in such a way that they don't break if you try to access<P>
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values beyond <A REL=DEFINITION HREF="../Body/v_multip.htm#multiple-values-limit"><B>MULTIPLE-VALUES-LIMIT</B></A>.<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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For proposal UNDEFINED, none.<P>
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For proposal <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, more care needs to be taken in implementing multiple<P>
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values.<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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For proposal <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, none.<P>
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For proposal UNDEFINED, loss of portability for some programs (probably<P>
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very rare).<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 implementation techniques for multiple values are<P>
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valid.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Hard to say.<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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Proposal <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is probably better from a purely theoretical point of view,<P>
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since it doesn't complicate the language by imposing arbitrary limits<P>
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on programs.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Loosemore prefers proposal <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, as long as it doesn't cause problems<P>
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for implementors.<P>
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-------<P>
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