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106 lines
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<H2>Issue DEFSTRUCT-PRINT-FUNCTION-INHERITANCE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss117.htm">DEFSTRUCT-PRINT-FUNCTION-INHERITANCE</A><P>
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<B>References:</B> CLtL p. 312-314<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit History:</B> V1, 5 Aug 1988, Sandra Loosemore<P>
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V2, 15 Sep 1988, Sandra Loosemore<P>
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V3, 7 Dec 1988, Masinter<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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CLtL doesn't make clear whether defstructs that :INCLUDE another<P>
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<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type and do not specify a :PRINT-FUNCTION inherit the<P>
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:PRINT-FUNCTION of the parent <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type. While it is stated on<P>
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page 314 that #S syntax is used if a :PRINT-FUNCTION is not specified,<P>
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the language on page 313 indicates that all operations on the parent<P>
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type will also work on objects of the child type. Because of the<P>
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ambiguity, existing implementations have gone both ways, and users<P>
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cannot depend on either #S syntax or the parent type's :PRINT-FUNCTION<P>
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being used.<P>
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<P>
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Proposal: <A HREF="iss117.htm">DEFSTRUCT-PRINT-FUNCTION-INHERITANCE:YES</A><P>
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<P>
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Clarify that <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> types which :INCLUDE another type but do not<P>
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specify an explicit :PRINT-FUNCTION inherit the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> print<P>
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function from the :INCLUDE'd type. A print function that uses the<P>
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default #S syntax (overriding any print function for the parent type)<P>
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can be specified by providing the :PRINT-FUNCTION option without an <P>
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argument.<P>
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<P>
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Supplying a :PRINT-FUNCTION in a <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> is equivalent<P>
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to defining an appropriate <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> on the <A REL=DEFINITION HREF="../Body/f_pr_obj.htm#print-object"><B>PRINT-OBJECT</B></A> generic<P>
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function.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Users typically specify a print function for a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type because<P>
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its slots will contain circular objects or large internal data<P>
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structures which are confusing when printed. Any <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type that<P>
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:INCLUDEs this type will also contain the same slots; it seems more<P>
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reasonable to inherit the parent's print function than to use the<P>
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default #S syntax.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Lucid Common Lisp makes structures inherit print functions.<P>
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VaxLisp uses #S syntax unless an explicit :PRINT-FUNCTION<P>
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is specified.<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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The changes to non-conforming implementations should be fairly minor<P>
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and localized.<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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It can't be any worse than the status quo.<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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An area of ambiguity in the language will be removed.<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 and VanRoggen support this proposal.<P>
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<P>
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The original version of the proposal did not <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> for a way to<P>
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force #S syntax to be used. This functionality was added to the<P>
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current version because there seemed to be general agreement that it<P>
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would be useful. Other alternatives would be to name the function<P>
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that does what the #S printer does, or to <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> a function that<P>
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returns the #S information as a list (as suggested by Waters) so users<P>
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can write their own.<P>
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<P>
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</PRE>
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