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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/iss112_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss113.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss114_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue DEFSTRUCT-COPIER Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss113.htm">DEFSTRUCT-COPIER</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A><P>
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<B>Related issues:<P>
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</B><B>Category:</B> CLARIFICATION, CHANGE<P>
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<B>Edit history:</B> V1, 10 May 90, Sandra Loosemore<P>
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V2, 13 May 90, Sandra Loosemore (update discussion)<P>
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V3, 29 May 90, Sandra Loosemore (more comments)<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 the :COPIER option to <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> in CLtL<P>
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states:<P>
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<P>
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The automatically defined copier function simply makes a new<P>
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<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> and transfers all components verbatim from the argument<P>
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into the newly created <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A>.<P>
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<P>
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This description doesn't make it clear whether the type of the<P>
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newly created <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> is copied from the argument, or is<P>
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determined by the copier function. This can make a difference<P>
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in the case where a copier function defined for one <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A><P>
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type is passed an argument that belongs to a subtype of the<P>
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original type. Does the result have the "name" of the subtype<P>
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and any additional slots present in the subtype but not the<P>
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parent type, or does it have the same "name" as the parent <P>
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type and only the slots inherited from that type?<P>
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<P>
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There are four proposals: ARGUMENT-TYPE, COPIER-TYPE,<P>
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UNSPECIFIED, and <A REL=DEFINITION HREF="../Body/f_rm_rm.htm#remove"><B>REMOVE</B></A>.<P>
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<P>
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<P>
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<B>Proposal (DEFSTRUCT-COPIER:ARGUMENT-TYPE):<P>
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</B><P>
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Clarify that the type of the result returned by copier functions <P>
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created by <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> is always exactly the same type as the <P>
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argument to the copier. That is, if the argument is of a<P>
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subtype of the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type for which the copier was defined,<P>
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then the result will also be of that subtype and include all<P>
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the slots of the subtype.<P>
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<P>
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Rationale for proposal ARGUMENT-TYPE:<P>
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Some people expect this behavior.<P>
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<P>
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<P>
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<B>Proposal (DEFSTRUCT-COPIER:COPIER-TYPE):<P>
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</B><P>
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Clarify that the type of the result returned by copier functions <P>
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created by <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> is always exactly the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type for <P>
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which the copier was defined. That is, if the argument is of a<P>
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subtype of the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type for which the copier was defined,<P>
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the result will be of the supertype only and include only those<P>
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slots that are present in the supertype.<P>
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<P>
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Rationale for proposal COPIER-TYPE:<P>
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Some people expect this behavior.<P>
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<P>
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<P>
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<B>Proposal (DEFSTRUCT-COPIER:UNSPECIFIED):<P>
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</B><P>
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Clarify that it is unspecified whether the type of the result <P>
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returned by copier functions created by <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> is the type<P>
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of the argument to the copier, or the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type for which the<P>
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copier function was defined. (In other words, the behavior of<P>
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copier functions is well-defined only when they are passed<P>
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arguments of the exact same type for which the copier was<P>
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defined.)<P>
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<P>
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Rationale for proposal UNSPECIFIED:<P>
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Some people don't know what behavior to expect.<P>
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<P>
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<P>
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<B>Proposal (DEFSTRUCT-COPIER:REMOVE):<P>
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</B><P>
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Remove the :COPIER option to <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A>.<P>
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<P>
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Rationale for proposal REMOVE:<P>
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The feature is so poorly specified it's already useless.<P>
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<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/m_defstr.htm#defstruct"><B>defstruct</B></A> foo a b)<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> (bar (:include foo)) c d)<P>
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(<A REL=DEFINITION HREF="../Body/s_setq.htm#setq"><B>setq</B></A> result (copy-foo (make-bar)))<P>
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<P>
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Under proposal ARGUMENT-TYPE,<P>
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(<A REL=DEFINITION HREF="../Body/f_tp_of.htm#type-of"><B>type-of</B></A> result) => BAR<P>
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<P>
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Under proposal COPIER-TYPE,<P>
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(<A REL=DEFINITION HREF="../Body/f_tp_of.htm#type-of"><B>type-of</B></A> result) => FOO<P>
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<P>
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Under proposal UNSPECIFIED,<P>
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(<A REL=DEFINITION HREF="../Body/f_tp_of.htm#type-of"><B>type-of</B></A> result) => either FOO or BAR<P>
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<P>
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Under proposal <A REL=DEFINITION HREF="../Body/f_rm_rm.htm#remove"><B>REMOVE</B></A>, users would have to define their own<P>
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copier functions to have whatever behavior they wanted.<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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Some Lisps now implement proposal ARGUMENT-TYPE and others implement <P>
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proposal COPIER-TYPE. Some Lisps also <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> a <A REL=DEFINITION HREF="../Body/f_cp_stu.htm#copy-structure"><B>COPY-STRUCTURE</B></A><P>
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function that can be used to make an exact duplicate of any<P>
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<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> object.<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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Small.<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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Code that relies on any particular behavior is probably not now<P>
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portable anyway. It doesn't seem like copiers are used very<P>
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much.<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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This part of the language specification remains vague.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Proposal <A REL=DEFINITION HREF="../Body/f_rm_rm.htm#remove"><B>REMOVE</B></A> would save space since there would be no<P>
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normally useless copier functions generated by <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A>.<P>
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It may be that copier functions defined by hand would be<P>
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considerably less efficient than a copier provided by the<P>
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implementation (which might make use of knowledge about the<P>
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internal layout of structures to do a block memory copy or the<P>
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like).<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 becomes more precise.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Leaving the behavior explicitly vague seems unnecessarily ugly.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Loosemore's ranking of the proposals from most to least<P>
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acceptable is: ARGUMENT-TYPE, <A REL=DEFINITION HREF="../Body/f_rm_rm.htm#remove"><B>REMOVE</B></A>, COPIER-TYPE,<P>
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UNSPECIFIED.<P>
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<P>
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BarMar has said he thinks the behavior should be left<P>
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unspecified.<P>
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<P>
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Moon has said he favors flushing the :copier feature. Barrett<P>
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also prefers flushing :copier.<P>
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<P>
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Perhaps we should consider adding <A REL=DEFINITION HREF="../Body/f_cp_stu.htm#copy-structure"><B>COPY-STRUCTURE</B></A> to the<P>
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standard?<P>
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-------<P>
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</PRE>
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