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93 lines
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<H2>Issue BUTLAST-NEGATIVE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss022.htm">BUTLAST-NEGATIVE</A><P>
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<B>Forum:</B> Editorial<P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> (p271), <A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A> (p271)<P>
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<B>Category:</B> CLARIFICATION/CHANGE<P>
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<B>Edit history:</B> 01-Mar-91, Version 1 by Pitman<P>
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<B>Status:</B> For X3J13 consideration<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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What happens if the second argument to <A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> and <A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A> is negative?<P>
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<P>
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<B>Proposal (BUTLAST:SHOULD-SIGNAL):<P>
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</B><P>
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Specify that if the second argument, N, to <A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> or <A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A> is negative, <P>
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then an error `should be signalled'.<P>
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<P>
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<B>Test Cases:<P>
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</B><P>
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1. (<A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> '(A B C) -2)<P>
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<P>
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2. (<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>LET</B></A> ((X (<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> 'A 'B 'C)))<P>
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(<A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A> X -2))<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Mostly this precludes implementations from adopting other, <P>
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variant interpretations.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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1. Symbolics Genera returns (A B C <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>).<P>
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<P>
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2. Symbolics Genera signals an error.<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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Small.<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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None. Portable programs cannot currently rely on any particular <P>
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behavior.<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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Users would be confused about what to expect because implementations might<P>
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adopt creative interpretations (such as Genera's) which are not supported<P>
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in other implementations and so become a barrier to portability.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Tighter spec.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Genera's alternate interpretation of <A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> is arguably aesthetic, but<P>
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does not extend well to <A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A>. The interpretation proposed here makes<P>
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<A REL=DEFINITION HREF="../Body/f_butlas.htm#butlast"><B>BUTLAST</B></A> and <A REL=DEFINITION HREF="../Body/f_butlas.htm#nbutlast"><B>NBUTLAST</B></A> more consistent.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Pitman supports this proposal.<P>
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</PRE>
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