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<H2>Issue DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE Writeup</H2>
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<PRE><B>Forum:</B> Cleanup<P>
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<B>Issue:</B> <A HREF="iss109.htm">DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE</A><P>
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<B>References:</B> CLtL page 316<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit history:</B> 20-Sep-88, Version 1, Peck<P>
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21-Sep-88, Version 2, Masinter, minor revisions<P>
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8-Jan-89, Version 3, Masinter<P>
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<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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Currently, <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> constructor functions can be either the default<P>
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constructor function, with *only* keyword arguments, or it can be a <P>
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so-called "By Order of Arguments" constructor function with explicitly<P>
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*no* keyword arguments. Other functions in Common Lisp allow a free<P>
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mix of required, optional, and keyword arguments. <P>
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<P>
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With the current restriction, it is necessary to hand code a function that<P>
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will accept optional and keyword arguments and parse the supplied-p<P>
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variables explicitly. Even so, it is not obvious to the casual programmer<P>
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how to <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> the same semantics as <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> does with respect to default<P>
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values and the <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> init-forms.<P>
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<P>
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Proposal: <A HREF="iss109.htm">DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE:ALLOW-KEY</A><P>
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<P>
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Allow <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> keyword arguments in constructor forms of DEFSTRUCTs<P>
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and the <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> token in addition to the <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&OPTIONAL</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A> and <A REL=DEFINITION HREF="../Body/03_da.htm#AMaux"><B>&AUX</B></A> arguments already allowed. Keyword arguments default<P>
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in a manner similar to that of <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&OPTIONAL</B></A> arguments: if no default<P>
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is supplied in the lambda-list then the slot initform is used;<P>
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otherwise the slot is not initialized -- its initial value is<P>
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undefined.<P>
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<P>
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If keyword arguments of the form ((key var) [default [svar]])<P>
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are specified, the "slot name" is matched with VAR (and not KEY).<P>
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<P>
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Additional arguments that do not correspond to slot names but<P>
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are merely present to supply values used in subsequent initialization <P>
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computations are allowed.<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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It should be possible to write forms like this:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> (foo (:constructor CREATE-FOO (a <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&optional</B></A> b (c 'sea)<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&key</B></A> (d 2)<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMaux"><B>&aux</B></A> e (f 'eff))))<P>
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(a 1) (b 2) (c 3) (d 4) (e 5) (f 6))<P>
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<P>
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(create-foo 10) => #S(foo a 10 b 2 c sea d 2 e nil f eff)<P>
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(create-foo 10 'bee 'see :d 'dee) => #S(foo a 10 b bee c see d dee e nil f eff)<P>
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<P>
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In the definition:<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> (frob (:constructor create-frob<P>
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(a <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&key</B></A> (b 3 have-b) (c-token 'c) <P>
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(c (<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>list</B></A> c-token (<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> have-b 7 2))))))<P>
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a b c)<P>
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<P>
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the c-token argument is used merely to supply a value used in the <P>
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initialization of the c slot. The "supplied-p" arguments of<P>
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keyword arguments might be of this form.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This is a logical extension of the specification which makes some<P>
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programming easier.<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 implementations signal an error if given <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> arguments or<P>
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arguments that are not slot names. The latest version of IIM Common <P>
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Lisp allows <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> arguments in this manner. Envos Medley<P>
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(Xerox Common Lisp) implements the proposal. <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 modifications to allow intermixed keywords and optionals in implementations<P>
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that don't already are likely simple. <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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No cost, this is upward compatible.<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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The current situation is non-intuitive and needless restrictive.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Much easier for users to write the constructor function they want.<P>
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Probably implementation code would be reduced, since this would no <P>
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longer be an error.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Minor improvement since it removes a needless restriction.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Possibly references to "By-position", "positional", and "By Order of<P>
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Arguments" constructor function might need to be changed to something else in<P>
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the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>. (They can still be called BOA-constructors, though, right? :-)<P>
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<P>
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Version 2 of this proposal was on the January 1989 ballot.<P>
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<P>
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<P>
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</PRE>
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