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<H2>Issue WITH-OUTPUT-TO-STRING-APPEND-STYLE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss364.htm">WITH-OUTPUT-TO-STRING-APPEND-STYLE</A><P>
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<P>
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<B>References:</B> CLtL, pages 331, 386<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> Version 1, 25-Mar-88 JonL<P>
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Version 2, 29-Mar-88 JonL (fix typos; comments by Daniels)<P>
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Version 3, 23-May-88 JonL (fix nits raised by Masinter)<P>
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Version 4, 23-May-88 JonL (change issue name -- only 1 proposal)<P>
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Version 5, 7-Jun-88 Masinter (more nits)<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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CLtL p386 says that FORMATting to a <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A>'d string should add<P>
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characters "as if by use of VECTOR-PUSH-EXTEND"; but CLtL p331 says that<P>
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<A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>WITH-OUTPUT-TO-STRING</B></A> will work "as if using <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A> if the<P>
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string is adjustable, and otherwise as if using VECTOR-PUSH". It's very<P>
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unlikely that the original authors of these parts intended differing<P>
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semantics for these two cases. Furthermore, the semantics for<P>
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<A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>WITH-OUTPUT-TO-STRING</B></A> permit the inconspicuous loss of characters<P>
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written to the string, since <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push"><B>VECTOR-PUSH</B></A> will just "drop on the floor"<P>
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any characters that would go beyond the end.<P>
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<P>
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<P>
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<B>Proposal (WITH-OUTPUT-TO-STRING-APPEND-STYLE:VECTOR-PUSH-EXTEND):<P>
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</B><P>
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Change the documentation of <A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>WITH-OUTPUT-TO-STRING</B></A> to be like that under <P>
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<A REL=DEFINITION HREF="../Body/f_format.htm#format"><B>FORMAT</B></A>. That is, replace the first sentence of the next-to-last paragraph <P>
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on CLtL p331 by:<P>
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"If *string* is specified, it must be a string with a fill pointer; <P>
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the output is incrementally appended to the string (as if by use of<P>
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<A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A>)."<P>
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<P>
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<P>
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<B>Test Case: <P>
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</B> (<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>let</B></A> ((str (<A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>make-array</B></A> 4 :element-type 'string-char :fill-pointer 0)))<P>
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(<A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>with-output-to-string</B></A> (s str) (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#princ"><B>princ</B></A> "You Luz, Bunkie!" s))<P>
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str)<P>
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CLtL behaviour will return "You "; proposed behaviour will signal an error.<P>
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<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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It's unlikely that the mention of <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push"><B>VECTOR-PUSH</B></A> in CLtL p331 was intended<P>
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to suggest that characters could be quietly "dropped on the floor". In<P>
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any case, there is no practical or theoretical reason to make <A REL=DEFINITION HREF="../Body/f_format.htm#format"><B>FORMAT</B></A> and<P>
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<A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>WITH-OUTPUT-TO-STRING</B></A> act differently on non-adjustable strings.<P>
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<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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VaxLisp 2.2 and Lucid 3.0 implement the proposal; Lucid 2.1 and earlier<P>
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versions implement CLtL. For <A REL=DEFINITION HREF="../Body/m_w_out_.htm#with-output-to-string"><B>WITH-OUTPUT-TO-STRING</B></A>, Xerox Common Lisp <P>
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implements CLtL. Symbolics Genera 7.2 implements the proposal.<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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Very 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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Virtually none.<P>
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<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Less special-casing in the semantics of "printing" to strings.<P>
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More conformity with naive expectations about printing to strings.<P>
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<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Minor impact.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Implementations may want to actually call <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push"><B>VECTOR-PUSH</B></A>, rather than<P>
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<A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A>, on non-adjustable string in order to test the<P>
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result -- nil means an overflow of the total length of the string; <P>
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thus they may signal an error more directly related to the problem,<P>
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rather than permitting <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A> to complain about a non-<P>
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adjustable array. But either way, the semantics is still that of<P>
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VECTOR-PUSH-EXTEND: when you get to the end of the string, adjustable <P>
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strings are extended, and non-adjustable strings cause error signals.<P>
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<P>
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It's perfectly acceptable to use <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A> with a non-adjustable<P>
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array. It's the error-signalling property of <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push-extend"><B>VECTOR-PUSH-EXTEND</B></A>, as opposed<P>
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to the "dropping on the floor" of <A REL=DEFINITION HREF="../Body/f_vec_ps.htm#vector-push"><B>VECTOR-PUSH</B></A>, that motivated this proposal.<P>
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<P>
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</PRE>
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