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<H2>Issue DEFSTRUCT-SLOTS-CONSTRAINTS-NAME Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss119.htm">DEFSTRUCT-SLOTS-CONSTRAINTS-NAME</A><P>
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<B>References:</B> CLtL p.308 & 86-003 p.4<P>
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<B>Category:</B> CLARIFICATION<P>
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<P>
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<B>Edit history:</B> Version 1 by Skona Brittain 05/13/88<P>
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Version 2, Masinter, 14-Sep-88<P>
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Version 3, Masinter, 23-Sep-88<P>
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Version 4, Masinter, 31-Oct-88<P>
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Version 5, Masinter, 12-Jan-89<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 case of two slots of a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> having the same name is not <P>
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discussed in CLtL. Is it allowed? The problem is that the<P>
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name of slot accessors and the keyword arguments of the <P>
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constructor function is determined only by the <A REL=DEFINITION HREF="../Body/f_symb_2.htm#symbol-name"><B>SYMBOL-NAME</B></A><P>
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of the slot designator; the meaning of slot accessors and<P>
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the constructor function is unspecified.<P>
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<P>
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<B>Proposal (DEFSTRUCT-SLOTS-CONSTRAINTS-NAME:DUPLICATES-ERROR):<P>
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</B><P>
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It is an error for two slots in a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> type to have the same symbol-name;<P>
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that is, the <A REL=DEFINITION HREF="../Body/f_symb_2.htm#symbol-name"><B>SYMBOL-NAME</B></A> of the slot names should not be <A REL=DEFINITION HREF="../Body/f_stgeq_.htm#stringEQ"><B>STRING=</B></A>.<P>
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This holds when they were both named directly by the same call to <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A><P>
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or when one is present by virtue of being in an included <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A>.<P>
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<P>
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The situation of expanding a <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> macro with a duplicate name "should<P>
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signal an error." (While not yet formally defined, the intent is that <P>
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the error signalling may occur when compiling a file that contains<P>
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duplicate names or when evaluating a <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> form with duplicate names<P>
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in an interpreter.)<P>
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<P>
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This proposal only affects the operation of the <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A><P>
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macro, and not the <A REL=DEFINITION HREF="../Body/t_stu_cl.htm#structure-class"><B>STRUCTURE-CLASS</B></A> or structures defined<P>
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with DEFCLASS><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> struc slot slot) would be an error. So would<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> (struc2 (:include struc1)) slot) if preceded by<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> struc1 slot).<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> struct package-1:slot package-2:slot) is also an<P>
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error. Slot accessors are interned in the current <A REL=DEFINITION HREF="../Body/v_pkg.htm#STpackageST"><B>*PACKAGE*</B></A><P>
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at the time of the evalution of the <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A>. <P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Since it would be difficult to prescribe reasonable behavior for<P>
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<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A>, it should be considered an error.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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In KCL, if two slots have the same name, no warning message is <P>
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given but mysterious behavior ensues. (Their default values are <P>
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both whatever is given for the second one, neither can be given a<P>
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different value via a call to the constructor function, only the <P>
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second one's value can be changed by <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</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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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.<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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Possible confusion.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Clarity.<P>
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<P>
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<B>Aethetics:<P>
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</B><P>
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Something that is not well-defined and leads to erratic behavior <P>
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should be explicitly considered an error.<P>
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<P>
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<B>Discussion: <P>
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</B><P>
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Although this issue was mentioned in Guy's original issues file, it has<P>
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not been officially discussed since. <P>
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<P>
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This issue was first circulated to X3J13 June 1988.<P>
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<P>
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This proposal does not address the issue of whether <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is a legitimate<P>
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slot name. There seems to be no current reason why it might be prohibitied.<P>
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<P>
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The compiler committee is proposing to address generally the issue <P>
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of how macro-expansion errors during <A REL=DEFINITION HREF="../Body/f_cmp_fi.htm#compile-file"><B>compile-file</B></A> might be caught and<P>
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turned into compiler warnings.<P>
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<P>
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<P>
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</PRE>
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