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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/iss093_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss094.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss095_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue DECLARE-FUNCTION-AMBIGUITY Writeup</H2>
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<PRE><B>Forum:</B> Cleanup<P>
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<B>Issue:</B> <A HREF="iss094.htm">DECLARE-FUNCTION-AMBIGUITY</A><P>
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<P>
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<B>References:</B> CLtL pp 43 (Table 4-1), 158-159<P>
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Issue <A HREF="iss175_m.htm">FUNCTION-TYPE</A> (passed X3J13/June 1988)<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> #1, 21 Sept 1988, Walter van Roggen<P>
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#2, 29 Sept 1988, Walter van Roggen (renamed; lessened ambiguity)<P>
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#3, 30-Sep-88, Masinter<P>
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#4, 5-Dec-88, Masinter (append Oct x3j13 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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CLtL permits confusing and ambiguous <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> declarations. One can say<P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> F (<A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>VECTOR</B></A> <A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A>) T))<P>
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to indicate that the function binding for F is of a certain type. Yet<P>
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one can also say<P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> X Y Z))<P>
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to indicate that the variables X, Y, and Z have values which are functions.<P>
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<P>
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The former is an abbreviation for<P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> (<A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>VECTOR</B></A> <A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A>) T) F))<P>
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The latter is an abbreviation for<P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>TYPE</B></A> <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> X Y Z))<P>
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<P>
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The ambiguity arises in a case like<P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> F <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> <A REL=DEFINITION HREF="../Body/a_string.htm#string"><B>STRING</B></A>))<P>
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However, it would be an error to declare the type of the constant <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> to be a<P>
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<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A>, so technically there is no ambiguity--there is just a peculiar<P>
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special case.<P>
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<P>
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Using the same declaration for two completely different purposes can lead<P>
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to confusion among people writing or reading such code.<P>
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<P>
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It is now more useful to declare that variables have a value which<P>
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is of type <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A>, since the type <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> has a more well-specified<P>
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meaning.<P>
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<P>
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Proposal (<A HREF="iss094.htm">DECLARE-FUNCTION-AMBIGUITY:DELETE-FTYPE-ABBREVIATION</A>)<P>
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<P>
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The declaration (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> name argtypes valtypes) is no longer permitted<P>
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to be an abbreviation for (<A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> argtypes valtypes) name).<P>
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<P>
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The declaration (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> var1 var2) would just be an abbreviation for<P>
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(<A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>TYPE</B></A> <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> var1 var2).<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Continuing to allow all the predefined atomic type specifiers as declaration<P>
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abbreviations for (<A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>TYPE</B></A> type var1 var2 ...) is simpler for users to understand.<P>
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In other words, all the normal type declarations describe variable bindings;<P>
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only the <A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> declaration describes function bindings. This is a more<P>
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uniform solution than making an exception to table 4-1.<P>
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<P>
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Since the old use of the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> declaration for function bindings was just<P>
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an abbreviation for the <A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> declaration, no expressivity is lost.<P>
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Furthermore one is able to say that a variable's value is of type <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A>,<P>
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something that hadn't been clearly possible without using the <A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>TYPE</B></A> declaration.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Many current implementations treat <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> declarations <P>
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only as abbreviations for <A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> declarations, primarily because<P>
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the proposal <A HREF="iss175_m.htm">FUNCTION-TYPE</A> has not been widely implemented.<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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Likely to be small to those implementations that heed these kinds of<P>
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declarations; none for those that don't.<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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Existing uses of the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> declaration for function bindings will need<P>
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to be changed to <A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> declarations.<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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People will continue to be confused by function declarations.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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A 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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<P>
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<B>Discussion:<P>
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</B><P>
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Making it clear that only <A REL=DEFINITION HREF="../Body/d_ftype.htm#ftype"><B>FTYPE</B></A> declarations describe function bindings will<P>
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make it easier to add new kinds of declarations that support incremental or<P>
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additional descriptions, as is needed for describing methods.<P>
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<P>
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Since all cases can be disambiguated after all, compatibility considerations<P>
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might deem this proposal to be unnecessary. However, making declarations<P>
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simpler and less confusing is possibly more important than compatibility.<P>
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<P>
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There is no consensus on the cleanup committee.<P>
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<P>
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Comments from October 1988 X3J13 meeting:<P>
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<P>
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... opposed but not vehemently--the incompatible change is gratuitous<P>
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... prefer to document the<P>
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issue rather than change it."<P>
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... a number of implementations accidentally implement<P>
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this incorrectly. They first check the type table and then handle<P>
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the elaborate function declaration style. But, as it happens,<P>
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they never reach the code for the second case because function is<P>
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in the type table!<P>
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... Having both styles is worse than having either one or the other.<P>
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<P>
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</PRE>
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