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96 lines
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<TITLE>CLHS: Issue COMMON-TYPE Writeup</TITLE>
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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/iss056_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss057.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss058_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue COMMON-TYPE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss057.htm">COMMON-TYPE</A><P>
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<P>
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<B>References:</B> CLtL p.35, p.76<P>
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<P>
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<B>Category:</B> CHANGE<P>
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<P>
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<B>Edit history:</B> Version 1, 20-Mar-89, by Moon<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 type COMMON is defined in a very peculiar way and does not seem to<P>
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be useful for anything. It can be extended by users using <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A>,<P>
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but not <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> nor <A REL=DEFINITION HREF="../Body/m_defcla.htm#defclass"><B>DEFCLASS</B></A>, but it cannot be extended by implementations.<P>
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Whether certain types such as <A REL=DEFINITION HREF="../Body/t_number.htm#number"><B>NUMBER</B></A> and <A REL=DEFINITION HREF="../Body/t_array.htm#array"><B>ARRAY</B></A> are subtypes of COMMON<P>
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is implementation-dependent. The goal of having the COMMON type was<P>
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probably to improve portability, but it is unclear how it could actually<P>
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be used in that way.<P>
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<P>
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<B>Proposal (COMMON-TYPE:REMOVE):<P>
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</B><P>
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Remove COMMON and COMMONP from the language.<P>
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<P>
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<B>Rationale:<P>
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</B> <P>
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Keeping the definition of COMMON accurate in the new specification, in<P>
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the face of changes elsewhere in the language such as the introduction<P>
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of CLOS and the possible introduction of character registries, is<P>
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difficult when no one is sure what COMMON is for. If no one uses COMMON,<P>
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it would be less work to just get rid of it.<P>
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<P>
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<B>Current practice:<P>
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</B> <P>
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Every implementation probably implements COMMON. Moon has never seen it<P>
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used except in a program to test whether its implementation matched CLtL.<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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None unless they are actually using COMMON.<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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Implementors would have to maintain COMMON. Users would have to try to<P>
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understand it, or figure out that they didn't care about it.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Simpler language.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Simpler language.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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None.<P>
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<P>
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</PRE>
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