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<H2>Issue SUBSEQ-OUT-OF-BOUNDS Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss332.htm">SUBSEQ-OUT-OF-BOUNDS</A><P>
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<P>
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<B>References:</B> :START and :END arguments (246-247), <A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A> (248)<P>
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<P>
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<B>Category:</B> CLARIFICATION<P>
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<P>
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<B>Edit history:</B> 24-Mar-88, Version 1 by Steele<P>
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29-Mar-88, Version 2 by Steele, in response to Moon's comments<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 descriptions of :START and :END arguments, and of <A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A>, do not<P>
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explicitly address the question of out-of-bounds indices. (The language on<P>
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page 246, "These arguments should be integer indices into the sequence," is<P>
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considered too vague on this point.)<P>
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<P>
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Also, the language on page 246 does not make clear whether the prohibition<P>
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against "start > end" applies to defaulted values as well as explicit<P>
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values, and does not specify clearly whether the default value for the<P>
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end argument is the allocated length or the active length.<P>
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<P>
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<P>
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<B>Proposal (SUBSEQ-OUT-OF-BOUNDS:IS-AN-ERROR):<P>
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</B><P>
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Specify that it is an error for the :START argument of any <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A><P>
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function, or the second argument to <A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A>, to be less than zero.<P>
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<P>
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Specify that it is an error for the :END argument of any <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> function,<P>
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or the third argument to <A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A>, to be greater than the active length of<P>
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the sequence in question (as returned by <A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>LENGTH</B></A>).<P>
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<P>
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Specify that the start value, after defaulting, must not be greater than<P>
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the end value, after defaulting.<P>
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<P>
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Specify that the default value for the end argument is the active length of<P>
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the sequence in question.<P>
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<P>
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This may be summarized as follows:<P>
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<P>
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Let X be the sequence within which indices are to be considered. Let S be<P>
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the :START argument of any <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> function, or the second argument to<P>
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<A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A>, whether explicitly specified or defaulted, through omission, to<P>
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zero. Let E be the :END argument of any <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> function, or the third<P>
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argument to <A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A>, whether explicitly specified or defaulted, through<P>
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omission or an explicitly passed <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> value, to the active length of X, as<P>
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returned by <A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>LENGTH</B></A>. It is an error if the condition (<A REL=DEFINITION HREF="../Body/f_eq_sle.htm#LTEQ"><B><=</B></A> 0 S E (<A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>LENGTH</B></A> X))<P>
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is not true.<P>
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<P>
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<B>Test Cases/Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A> "Where's the beef?" -1 5) might be assumed to be "Where" or " Where".<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A> "Where's the beef?" -3 -3) might be assumed to be "".<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A> "Where's the beef?" 16 18) might be assumed to be "?" or "? ".<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_subseq.htm#subseq"><B>SUBSEQ</B></A> "Where's the beef?" 10000 10000) might be assumed to be "".<P>
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<P>
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Under this proposal each of these situations is an error, and portable<P>
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programs may not rely on their behavior.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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We don't want code indexing off the ends of arrays.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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KCL interpreted and compiled code signals an error.<P>
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<P>
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Symbolics Common Lisp interpreted and compiled code signals an error; the<P>
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compiler also issued an out-of-range warning (possible because the<P>
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arguments were all constant).<P>
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<P>
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Lucid Common Lisp interpreted and compiled code signals an error.<P>
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<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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None.<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 who depended on some specific implementation behavior in these cases<P>
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may find that their pragmatically unportable code is now officially<P>
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unportable.<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.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Removal of a small but important ambiguity in the spec.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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It seems cleaner not to allow indexing off the end of an array, and<P>
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by extension not allow it for any sequence.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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This merely clarifies the original intent of the passage on page 246.<P>
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<P>
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</PRE>
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