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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss287_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss288.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss289_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue READ-SUPPRESS-CONFUSING Writeup</H2>
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<PRE><B>Forum:</B> Public Review<P>
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<B>Issue:</B> <A HREF="iss288.htm">READ-SUPPRESS-CONFUSING</A><P>
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<B>References:</B> Margolin's PR comment #2 (X3J13/92-1402)<P>
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Margolin's PR comment #18 (X3J13/92-1418)<P>
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Flanagan's PR comment #7 (X3J13/92-1907)<P>
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Philpot's PR comment #1 (X3J13/92-2901)<P>
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Barrett's PR comment #9 (X3J13/92-3109), part 2<P>
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<A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> (X3J13/92-102 pp.23-21..22)<P>
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Sharpsign P (X3J13/92-102 sec.2.4.8.14)<P>
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Sharpsign C (X3J13/92-102 sec.2.4.8.11)<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit history:</B> 5 Jan 1993, Version 1 by Margolin<P>
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22 May 1993, Version 2 by Loosemore (delete bogus reference<P>
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to ":" and "@" in dispatching macro syntax)<P>
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<B>Status:</B> Proposal GENERALIZE passed 9-2 on letter ballot 93-302<P>
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<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 description of <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> is confusing and incomplete. By<P>
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specifying the behavior of particular read syntaxes rather than a simple,<P>
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general strategy, some syntaxes (#C, #P, and#') have unintentionally been<P>
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omitted. And the wording of the description of the behavior of #(...) is<P>
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unclear as to whether the vector syntax is parsed and ignored or a vector<P>
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is actually constructed (there's a subtle difference between the CLtL and<P>
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dpANS wording, due to the lack of a pair of parentheses in the dpANS).<P>
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<P>
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A strict reading of the current description of <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> implies<P>
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that the reader must still construct objects in response to these<P>
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syntaxes (as well as user-defined reader macros that don't check the<P>
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variable). This is pretty wasteful considering that <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A><P>
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exists primarily to support #+ and #-, which discard what they read with<P>
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<A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> true anyway.<P>
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<P>
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In the case of #P, the lack of specification of its <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A><P>
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behavior is a real annoyance, because some implementations have extended<P>
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it to allow non-strings to follow it. Users expect to be able to hide<P>
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such extensions using #+, but this isn't valid.<P>
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<P>
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<B>Proposal (READ-SUPPRESS-CONFUSING:GENERALIZE):<P>
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</B><P>
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1) Replace the portion of the Description of <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> from<P>
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"Extended tokens" through the entry for "#A, #S, #:" with:<P>
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<P>
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When <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> is true, <A REL=DEFINITION HREF="../Body/f_rd_rd.htm#read"><B>READ</B></A>, <A REL=DEFINITION HREF="../Body/f_rd_rd.htm#read-preserving-whitespace"><B>READ-PRESERVING-WHITESPACE</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_rd_del.htm#read-delimited-list"><B>READ-DELIMITED-LIST</B></A>, and <A REL=DEFINITION HREF="../Body/f_rd_fro.htm#read-from-string"><B>READ-FROM-STRING</B></A> all return a first value of<P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> when they complete successfully; however, they continue to parse<P>
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the representation of an object in the normal way, in order to skip<P>
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over the object, and continue to indicate <A REL=DEFINITION HREF="../Body/e_end_of.htm#end-of-file"><B>end-of-file</B></A> in the normal<P>
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way. Except as noted below, any <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> macro character (including<P>
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dispatching macro sub-characters) that is defined to read a following<P>
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object or token will do so, but not signal an error if the object<P>
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read is not of an appropriate type or syntax. Standard syntaxes and<P>
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macro characters will not construct any new objects (e.g. when<P>
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reading the representation of a symbol, no symbol will be constructed<P>
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or interned).<P>
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<P>
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Dispatching macro characters (including #)<P>
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<P>
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Dispatching macro characters continue to parse an infix numerical<P>
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argument, and invoke the<P>
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dispatch function. The <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> # dispatching macro characters<P>
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do not enforce any constraints on the value or existence of the<P>
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numerical argument.<P>
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<P>
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2) Create a Notes section that recommends that user-defined and<P>
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implementation-defined macro characters behave analogously to the<P>
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<A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> macro character, i.e. when <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> is true they should<P>
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ignore type errors when reading a following object and dispatching macro<P>
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characters should allow <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> as an infix parameter.<P>
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<P>
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3) Change the specifications of #C and #P to specify that they are<P>
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followed by the representation of an object. Then specify how they are<P>
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interpreted when those objects are a list of two real numbers (for #C) or<P>
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a string (for #P).<P>
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<P>
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<B>Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>let</B></A> ((<A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*read-suppress*</B></A> t))<P>
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(<A REL=DEFINITION HREF="../Body/m_dolist.htm#dolist"><B>dolist</B></A> (<A REL=DEFINITION HREF="../Body/a_string.htm#string"><B>string</B></A> '("#(foo bar baz)" "#P(:type :lisp)" "#c1.2" "foo"))<P>
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(<A REL=DEFINITION HREF="../Body/m_when_.htm#unless"><B>unless</B></A> <P>
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(<A REL=DEFINITION HREF="../Body/m_ignore.htm#ignore-errors"><B>ignore-errors</B></A><P>
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(<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>progn</B></A> (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>print</B></A> (<A REL=DEFINITION HREF="../Body/f_rd_fro.htm#read-from-string"><B>read-from-string</B></A> <A REL=DEFINITION HREF="../Body/a_string.htm#string"><B>string</B></A>))<P>
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t))<P>
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(<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>print</B></A> '<A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>error</B></A>))))<P>
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could print<P>
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#(<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> <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>)<P>
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<A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A><P>
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<A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A><P>
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<P>
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<B>Test Cases:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>let</B></A> ((<A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*read-suppress*</B></A> t))<P>
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(<A REL=DEFINITION HREF="../Body/f_mapc_.htm#mapcar"><B>mapcar</B></A> #'<A REL=DEFINITION HREF="../Body/f_rd_fro.htm#read-from-string"><B>read-from-string</B></A><P>
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'("#(foo bar baz)" "#P(:type :lisp)" "#c1.2")))<P>
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-> (<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> <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>)<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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1) Specifies a general behavior for <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> that is applicable to<P>
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all reader syntaxes that are described in terms of the representations of<P>
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objects.<P>
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<P>
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2) Encourages implementors and users to extrapolate from this <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A><P>
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behavior.<P>
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<P>
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3) Specifies #P and #C in such a way that change 1 applies.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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For the Example form, Genera 8.1 prints <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>, <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> (it seems to<P>
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implement the proposal); Lucid 4.1 and CMU CL 16d print <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A>, NIL;<P>
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WCL 2.1 prints #(<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> <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>), <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A>, <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A>.<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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Should be minor.<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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When <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> is bound via #+ or #-, the change should be upward<P>
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compatible, since any objects constructed by the reader will not be<P>
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visible. Direct users of <A REL=DEFINITION HREF="../Body/v_rd_sup.htm#STread-suppressST"><B>*READ-SUPPRESS*</B></A> may notice the difference, but<P>
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it seems unlikely that anyone is depending on any of the current<P>
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behaviors.<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 will have to avoid using some implementation-dependent syntaxes in<P>
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#P macros.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Minor.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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More ability to include implementation-dependent syntax extensions in<P>
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portable source code.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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A general description of behavior is better than lots of specific cases.<P>
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<P>
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<B>Editorial Impact:<P>
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</B><P>
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Minor. The changes are confined to the three sections mentioned above.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Barmar originally wanted to specify that <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> dispatching macros<P>
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would parse the numerical argument but ignore it (always passing <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> to<P>
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the dispatch function). Flanagan convinced him to leave that part as it<P>
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was originally (the numeric argument is parsed but <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> dispatch<P>
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functions don't enforce restrictions on it). It doesn't seem to be a big<P>
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deal either way.<P>
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<P>
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</PRE>
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