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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/iss288_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss289.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss290_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue READER-ERROR Writeup</H2>
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<PRE><B>Forum:</B> Cleanup<P>
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<B>Issue:</B> <A REL=DEFINITION HREF="../Body/e_rder_e.htm#reader-error"><B>READER-ERROR</B></A><P>
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<B>References:</B> Chapter 3, "Syntax"<P>
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Section 2.2, "Types"<P>
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<B>Category:</B> ADDITION, CHANGE<P>
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<B>Edit History:</B> V1, 23 Oct 1989, Sandra Loosemore<P>
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V2, 02 Nov 1989, Sandra Loosemore<P>
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(change supertypes, update discussion)<P>
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V3, 10 Jul 1990, David Moon (include amendment from meeting)<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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Chapter 3 of the current draft is not consistent about the types of<P>
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errors that must be signalled by the reader. In most places where it<P>
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specifies that an error is to be signalled, it does not indicate a<P>
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particular type of error, but in at least one situation it requires<P>
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the error to be of type <A REL=DEFINITION HREF="../Body/e_progra.htm#program-error"><B>PROGRAM-ERROR</B></A>, and in other cases it requires<P>
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the error to be of type <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A>.<P>
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<P>
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None of the <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A> subtypes described in section 2.2 really seem<P>
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appropriate for syntactic errors detected by the reader. In<P>
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particular, the description of <A REL=DEFINITION HREF="../Body/e_progra.htm#program-error"><B>PROGRAM-ERROR</B></A> implies that its purpose<P>
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is for errors relating to program syntax which are detectable during<P>
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evaluation or compilation rather than read-time errors. It seems<P>
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especially inappropriate to use <A REL=DEFINITION HREF="../Body/e_progra.htm#program-error"><B>PROGRAM-ERROR</B></A> for this purpose since<P>
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the reader can be used to read things other than programs. Likewise,<P>
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<A REL=DEFINITION HREF="../Body/e_stm_er.htm#stream-error"><B>STREAM-ERROR</B></A> appears to have been intended to be used for errors<P>
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relating to character-level transactions on the stream rather than<P>
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lexical analysis or parsing.<P>
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<P>
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<P>
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<B>Proposal (READER-ERROR:NEW-TYPE):<P>
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</B><P>
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Add a new type specifier, <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A>. This is a subtype of the<P>
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types <A REL=DEFINITION HREF="../Body/e_stm_er.htm#stream-error"><B>STREAM-ERROR</B></A>, <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A>, <A REL=DEFINITION HREF="../Body/e_seriou.htm#serious-condition"><B>SERIOUS-CONDITION</B></A>, <A REL=DEFINITION HREF="../Body/e_cnd.htm#condition"><B>CONDITION</B></A>, and T. The<P>
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type <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> consists of serious conditions that relate to<P>
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lexical analysis (the building and interpretation of tokens) and<P>
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parsing (errors in reader macro syntax) by the Lisp reader.<P>
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<P>
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Since <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> is a subtype of <A REL=DEFINITION HREF="../Body/e_stm_er.htm#stream-error"><B>STREAM-ERROR</B></A>, objects of this type<P>
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inherit a <A REL=DEFINITION HREF="../Body/t_stream.htm#stream"><B>STREAM</B></A> slot, which can be accessed using the function<P>
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<A REL=DEFINITION HREF="../Body/f_stm_er.htm#stream-error-stream"><B>STREAM-ERROR-STREAM</B></A>.<P>
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<P>
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Change the discussion in chapter 3 to specify that the type of errors<P>
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signalled by the reader is <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A>. There are numerous places<P>
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that would be affected.<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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The general policy that has been followed in other areas of the<P>
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language where errors must, should, or might be signalled is to<P>
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specify a particular subtype of <A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>ERROR</B></A> which covers that <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> of<P>
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errors. Doing the same for reader errors would be consistent with the<P>
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overall policy, but none of the existing error types seem appropriate<P>
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for reader-related errors.<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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LispWorks (from Harlequin) reportedly already uses such a condition.<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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Trivial.<P>
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<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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Probably no user programs rely on the reader signalling any particular<P>
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type of error yet.<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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Increased consistency in the language design.<P>
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Ability to set up handlers specifically for reader errors.<P>
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<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Pitman says:<P>
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Our later comments will show that i would like this condition to be<P>
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called <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> so that it doesn't seem to be useful only for <P>
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READ-related problems but can in fact be used for other parser-style<P>
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applications (e.g., a FORTRAN parser, a user-written English-language<P>
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interface, etc.).<P>
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<P>
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The initial version of this proposal specified that <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> was<P>
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disjoint from (rather than a subtype of) <A REL=DEFINITION HREF="../Body/e_stm_er.htm#stream-error"><B>STREAM-ERROR</B></A>. This was<P>
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changed because there was a suggestion that some conditions should be<P>
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both a <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> and a <A REL=DEFINITION HREF="../Body/e_stm_er.htm#stream-error"><B>STREAM-ERROR</B></A>. Making it a subtype also allows<P>
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it to inherit the <A REL=DEFINITION HREF="../Body/t_stream.htm#stream"><B>STREAM</B></A> slot and the <A REL=DEFINITION HREF="../Body/f_stm_er.htm#stream-error-stream"><B>STREAM-ERROR-STREAM</B></A> accessor.<P>
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<P>
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Prior to version 3 the condition <A REL=DEFINITION HREF="../Body/e_parse_.htm#parse-error"><B>PARSE-ERROR</B></A> was named <A REL=DEFINITION HREF="../Body/e_rder_e.htm#reader-error"><B>READER-ERROR</B></A>.<P>
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</PRE>
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