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<H2>Issue ARGUMENT-MISMATCH-ERROR-AGAIN Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss010.htm">ARGUMENT-MISMATCH-ERROR-AGAIN</A><P>
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<B>Forum:</B> Cleanup<P>
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<B>References:</B> <A HREF="iss012.htm">ARGUMENT-MISMATCH-ERROR</A>, draft 10.156 pp.3-44, 3-48<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit history:</B> 9-Dec-91, Version 1 by Moon<P>
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<B>Status:</B> Accepted 10-Dec-91; 10-1<P>
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<P>
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<B>Problem description:<P>
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</B> <P>
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<A HREF="iss012.htm">ARGUMENT-MISMATCH-ERROR</A> left out two cases of argument mismatch error:<P>
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(1) a mismatch between a destructuring <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list and the tree being destructured;<P>
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(2) an odd number of arguments supplied to the keyword parameters.<P>
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<P>
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In the current specification, these are both "the consequences are undefined."<P>
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This is inconsistent with the handling of the other five argument mismatch errors.<P>
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<P>
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<B>Proposal (ARGUMENT-MISMATCH-ERROR-AGAIN:CONSISTENT):<P>
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</B><P>
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Add two new subsections to section 3.5, with wording analogous to the subsections<P>
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already present, to cover these cases, as follows:<P>
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<P>
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(1) After the existing sentence "The pattern and the \term{macro form} must have<P>
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compatible \term{tree structure}; that is, their \term{tree structure} must be<P>
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equivalent, or it must differ only in that some \term{leaves} of the pattern<P>
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match \term{non-atomic} \term{objects} of the \term{macro form}." add "In a safe<P>
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call, an error of type <A REL=DEFINITION HREF="../Body/e_progra.htm#program-error"><B>program-error</B></A> must be signaled; otherwise, the situation<P>
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has undefined consequences." followed by the boilerplate about the exact point of<P>
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the signal.<P>
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<P>
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(2) Add the following: "An odd number of arguments must not be supplied for the<P>
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keyword parameters. In a \term{safe} \term{call}, an error \oftype{program-error}<P>
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must be signaled unless either \keyref{allow-other-keys} was used in the<P>
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\term{function}'s definition, or ``\f{:allow-other-keys \term{true}}'' was used<P>
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in the \term{call}; otherwise, the \term{situation} has undefined consequences."<P>
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followed by the boilerplate about the exact point of the signal. <P>
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<P>
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Move some text now on page 3-44 to the new subsection of 3.5 dealing with destructuring.<P>
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<P>
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<B>Rationale:<P>
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</B> <P>
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Consistency.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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CLtL did not <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> this level of error checking, so it's entirely <P>
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likely that there are implementations which do not conform.<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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The cost of adding additional error checking in some implementations.<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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More robust code.<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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Less safety when users use defmacro; inconsistency in the specification.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None.<P>
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</PRE>
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