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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/iss120_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss121.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss122_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue DEFTYPE-DESTRUCTURING Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss121.htm">DEFTYPE-DESTRUCTURING</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A><P>
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Related issues: <A HREF="iss122.htm">DEFTYPE-KEY</A><P>
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<B>Category:</B> CLARIFICATION, CHANGE<P>
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<B>Edit history:</B> V1, 23 May 90, Sandra Loosemore<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 specification of <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> is not clear on the issue of whether<P>
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it is supposed to support destructuring. The description in CLtL<P>
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twice compares its syntax to that of <A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A>, leading some<P>
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people to think that it does support destructuring. However,<P>
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since destructuring is not explicitly mentioned, other people<P>
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think it does not.<P>
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<P>
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There are two proposals, YES and NO.<P>
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<P>
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<B>Proposal (DEFTYPE-DESTRUCTURING:YES):<P>
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</B> <P>
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Clarify that <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> does support destructuring of the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list.<P>
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The lambda-list syntax for <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> is identical to that of<P>
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<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A>.<P>
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<P>
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Rationale for proposal YES:<P>
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Some people think this is the way it was really supposed to<P>
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work, and that supporting destructuring makes the syntax of <P>
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<A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> more consistent with other defining macros.<P>
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<P>
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<B>Proposal (DEFTYPE-DESTRUCTURING:NO):<P>
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</B><P>
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Clarify that <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> does not support destructuring of the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A><P>
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list. <P>
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<P>
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Rationale for proposal NO:<P>
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This requires minimal changes for implementors. The use of<P>
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destructuring with <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> is rare and since some<P>
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implementations do not support it now, code that relies on<P>
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it working is already nonportable.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Spice Lisp implemented a destructuring <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A>, and a number<P>
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of implementations have copied this behavior. Other<P>
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implementations do not support destructuring in <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</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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For proposal YES, fairly small, since every implementation<P>
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already has support for destructuring for other parts of<P>
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the language.<P>
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<P>
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For proposal NO, none. Implementations that now support <P>
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destructuring can continue to do so as an extension.<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 for either proposal. Code that relies on destructuring <P>
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with <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> is already not portable, but on the other hand<P>
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adding destructing support shouldn't break code that doesn't<P>
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use it.<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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Continuing vagueness in this part of the language specification.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Probably insignificant.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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This part of the language specification is made more clear.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Seems to be a matter of personal taste.<P>
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<P>
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<B>Discussion:<P>
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</B>-------<P>
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</PRE>
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