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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/iss153_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss154.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss155_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue EXTENSIONS-POSITION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss154.htm">EXTENSIONS-POSITION</A><P>
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<B>References:</B> Chapter 1, Working draft of <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A><P>
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<B>Category:</B> Clarification<P>
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Related Issues: CONFORMANCE-POSITION, IF-BODY, ERROR-TERMINOLOGY, EXTRA-SYNTAX,<P>
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EXTRA-OPTIONAL-KEYWORD-ARGUMENTS, UNSPECIFIED-DATATYPES,<P>
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<A HREF="iss227.htm">MACRO-AS-FUNCTION</A>, UNSOLCITED-MESSAGES, <A HREF="iss156.htm">EXTRA-RETURN-VALUES</A><P>
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<B>Edit history:</B> 12-DEC-88, Version 1 by Chapman<P>
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20-DEC-88, Version 2 by Chapman<P>
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9-JAN-89, Version 3 by Chapman<P>
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10-JAN-89, Version 4 by Chapman<P>
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2-FEB-89, Version 5 by Chapman<P>
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24-FEB-89, Version 6 by Chapman (added RPG's comment)<P>
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10-MAR-89, Version 7 by Chapman (added discussion)<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 is the definition of a language extension?<P>
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What effect does a language extension have on a conforming program? <P>
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What obligation does an implementation have to warn the user that an <P>
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extension is being used?<P>
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<P>
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Presumably the only thing that defining it as an extension can mean from<P>
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CL's point of view is `initially defining' it as an extension. Whether<P>
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an implementation permits redefinition of an extension is between that<P>
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implementation and its users and beyond the scope of Common Lisp. For<P>
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example, it is common practice to redefine some kinds of system functions<P>
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in Genera -- to extend the system in interesting ways, to fix bugs, etc.<P>
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<P>
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<P>
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Proposal (<A HREF="iss154.htm">EXTENSIONS-POSITION:DOCUMENTATION</A>)<P>
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<P>
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The <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> document should define a language extension to be<P>
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any implementation-supplied tool that isn't explicitly defined<P>
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in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>. This includes facilities added to tools defined<P>
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in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>.<P>
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The <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> document should levy the following requirement on a <P>
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conforming implementation's documentation:<P>
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The documentation that accompanies a conforming implementation should clearly<P>
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state which parts of the implementation are extensions. <P>
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<P>
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<P>
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If the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> says that "the results are unspecified", and an<P>
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implementation specifies the results, this an extension in the<P>
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sense that if the correct behavior of a program depends on the results,<P>
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only implementations with the same extension will execute the program<P>
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correctly.<P>
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<P>
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In places where the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> says that "an implementation may be extended",<P>
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this implies that a conforming, but probably non-portable, program can<P>
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be written using the implementation's extension.<P>
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<P>
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Proposal (EXTENSIONS-POSITION:DISABLE)<P>
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<P>
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Same as <A HREF="iss154.htm">EXTENSIONS-POSITION:DOCUMENTATION</A> except that<P>
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an implementation is required to have a way to disable its extensions, so<P>
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that a programmer can be told when he is using a feature that might<P>
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affect his program's portability. <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 <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> should contain information about language extensions<P>
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since most implementations have extended the language.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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CLtL allows any extension, provided that it doesn't alter the behavior<P>
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of a program that only uses what is specified in CLtL. In particular,<P>
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any situation that "is an error" (either explicitly or implicitly) is a<P>
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potential area for extension.<P>
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<P>
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<P>
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<B>Adoption Cost:<P>
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</B> <P>
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Vendors will have to improve their documentation<P>
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to list all their extensions. Vendors will have to go through their<P>
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implementation and determine what is or isn't an extension.<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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This definition will <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> a basis for proper understanding of <P>
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the error terminology used in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>. The implementation<P>
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documentation requirement will aid the user in producing portable code.<P>
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<P>
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<B>Conversion Cost:<P>
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</B> <P>
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None.<P>
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<P>
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<B>Aesthetics:<P>
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</B> <P>
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None.<P>
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<P>
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<B>Discussion:<P>
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</B>Masinter says:<P>
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It seems to be a constraint on "documentation" rather than "implementation"<P>
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if you turn the accidental behavior of (<A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>CAR</B></A> T) into a "feature" of your<P>
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implementation. We might want to disallow such an extension as "conforming<P>
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to the standard". An implementation which had such an extension might<P>
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conform, even if the extension did not conform.<P>
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<P>
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RPG says:<P>
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I favor remaining mute on this topic in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>.<P>
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<P>
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Moon says:<P>
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"I favor <A HREF="iss154.htm">EXTENSIONS-POSITION:DOCUMENTATION</A>.<P>
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<P>
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I oppose EXTENSIONS-POSITION:DISABLE because it mandates a<P>
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particular development environment feature, but Common Lisp<P>
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has avoided saying anything about development environments<P>
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since that is an area of extreme controversy.<P>
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<P>
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Gabriel's position of standing mute would be okay with me."<P>
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</PRE>
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