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<TITLE>CLHS: Issue INITIALIZATION-FUNCTION-KEYWORD-CHECKING Writeup</TITLE>
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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/iss196_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss197.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss198_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue INITIALIZATION-FUNCTION-KEYWORD-CHECKING Writeup</H2>
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<PRE><B>Status:</B> Passed v4 (8-1), as amended, Dec-91; v5 reflects the amendment<P>
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<B>Issue:</B> <A HREF="iss197.htm">INITIALIZATION-FUNCTION-KEYWORD-CHECKING</A><P>
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Reference: Draft 10.156<P>
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7.2.1.1 Initialization Arguments (p.7-6)<P>
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(which corresponds to CLtL-II 28.1.9.1, p.802)<P>
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7.2.1.2 Declaring the Validity of Initialization Arguments<P>
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(p.7-7) (which corresponds to CLtL-II 28.1.9.2, p.803)<P>
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7.3.5 Keyword Arguments in Generic Functions and Methods<P>
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(p.7-20) (which corresponds to CLtL-II 28.1.6.5, p.792)<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit History:</B> Version 1, 10/2/91, Kim Barrett<P>
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Version 2, 10/8/91, Kim Barrett (Moon & JonL comments)<P>
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Version 3, 10/11/91, Kim Barrett (more Moon comments)<P>
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Version 4, 11/20/91, Kim Barrett<P>
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(clarify :allow-other-keys, make references to 10.156 draft)<P>
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Version 5, 11-Feb-92, Kent Pitman<P>
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("has no effect on" => "suppresses" per X3J13 meeting amendment)<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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A careful reading of 88-002R indicates that users must specify<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in the lambda-lists of methods for initialization functions<P>
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if the <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> also specifies <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A>, else risk having the generic function<P>
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keyword argument checking signal an error due to the presence of "slot<P>
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filling" initargs. (An error might not occur due to the presence of other<P>
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methods which already specify <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> + <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A>).<P>
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<P>
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Several implementors (and presumably users too) have misread the specification<P>
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as saying that <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in an applicable initialization <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> means<P>
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that initarg validation is suppressed for those protocols in which the <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A><P>
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takes part.<P>
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<P>
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The result is that the need for and meaning of <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in<P>
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initialization methods differs significantly from one implementation to<P>
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another. <P>
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<P>
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<B>Proposal:<P>
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</B><P>
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1. Change the syntax of the following generic functions to specify both <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A><P>
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and &ALLOW-OTHER-KEYS:<P>
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<P>
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<A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> &key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_init_i.htm#initialize-instance"><B>INITIALIZE-INSTANCE</B></A> object &key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_mk_ins.htm#make-instance"><B>MAKE-INSTANCE</B></A> <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> &key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_reinit.htm#reinitialize-instance"><B>REINITIALIZE-INSTANCE</B></A> object &key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_shared.htm#shared-initialize"><B>SHARED-INITIALIZE</B></A> object slot-names &key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_update.htm#update-instance-for-different-class"><B>UPDATE-INSTANCE-FOR-DIFFERENT-CLASS</B></A> previous current<P>
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&key &allow-other-keys<P>
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<A REL=DEFINITION HREF="../Body/f_upda_1.htm#update-instance-for-redefined-class"><B>UPDATE-INSTANCE-FOR-REDEFINED-CLASS</B></A> object<P>
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added-slots discarded-slots<P>
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property-list<P>
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&key &allow-other-keys<P>
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<P>
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It is of course possible to retain "&rest initargs" in the syntax description<P>
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when needed in the corresponding function description.<P>
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<P>
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2. Clarify that the presence of <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in the lambda-list of an<P>
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applicable <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> relevant to the specified initarg validity checking for an<P>
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initialization protocol suppresses the validity checking.<P>
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<P>
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3. Clarify that a true value for the initialization argument :ALLOW-OTHER-KEYS<P>
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disables initialization argument validation. Make the following changes Draft<P>
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10.156 in order make this clarification.<P>
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<P>
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a. Remove the last sentence of the second paragraph of section 7.2.1.1<P>
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(p.7-6), which says<P>
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<P>
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"Error checking of initialization argument names is disabled if the keyword<P>
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argument pair whose keyword is :ALLOW-OTHER-KEYS and whose value is non-NIL<P>
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appears in the initialization argument list."<P>
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<P>
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b. Replace the last sentence of section 7.2.1.2 (p.7-7), which says<P>
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<P>
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"The meaning of:ALLOW-OTHER-KEYS is the same here as when it is passed to an<P>
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ordinary function."<P>
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<P>
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with<P>
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<P>
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"Validity checking of initialization arguments is disabled if the value of<P>
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the initialization argument :ALLOW-OTHER-KEYS is true."<P>
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<P>
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<B>Editorial impact:<P>
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</B><P>
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The syntax entries for seven functions need to be modified.<P>
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<P>
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A small change to "Declaring the Validity of Initialization Arguments" is<P>
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needed in order to clarify the handling of <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in initialization <P>
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methods.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Since <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> needs to be specified somewhere if <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> is specified<P>
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by any of the applicable methods, it is better to have the generic function<P>
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specify it, rather than requiring programmers to remember to include it in<P>
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every initialization <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> that also specifies <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A>.<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_defcla.htm#defclass"><B>defclass</B></A> test ()<P>
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((slot :initarg :slot-filler))<P>
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)<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defmet.htm#defmethod"><B>defmethod</B></A> <A REL=DEFINITION HREF="../Body/f_init_i.htm#initialize-instance"><B>initialize-instance</B></A> :after ((x test) <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&key</B></A>) <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>nil</B></A>)<P>
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(<A REL=DEFINITION HREF="../Body/f_mk_ins.htm#make-instance"><B>make-instance</B></A> 'test :slot-filler 5)<P>
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-> currently might signal an error<P>
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-> returns instance of test under proposed change.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Both Lucid 4.0 and IIM 3.4 specify only <A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A> in the lambda-lists of the<P>
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listed generic functions and the specified methods for those functions, with<P>
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the implication that user defined methods which specify <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> must also specify<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in order to inhibit keyword argument checking by the generic<P>
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function from signaling an error due to the presence of "slot filling"<P>
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initargs. However, because generic function keyword checking has not yet been<P>
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implemented in either Lucid 4.0 or IIM 3.4, users may not be aware of this<P>
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requirement. In fact, in IIM 3.4 there are system-supplied initialization<P>
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methods that fail to meet this requirement, so there are some implementors who<P>
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aren't aware of this problem either.<P>
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<P>
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Franz Allegro 4.0 appears to have the same behavior as described above for<P>
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Lucid 4.0 and IIM 3.4. In particular, generic function keyword checking has<P>
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not yet been implemented.<P>
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<P>
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Symbolics CLOS and Chestnut's Lisp-to-C translator both specify <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> and<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in the lambda-lists of the listed generic functions. In<P>
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addition, if an applicable <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for one of the protocols specifies<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> then the specified initarg checking for the protocol is not<P>
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performed.<P>
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<P>
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Macintosh Common Lisp 2.0b3c4 has "&KEY &ALLOW-OTHER-KEYS" in the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list<P>
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for <A REL=DEFINITION HREF="../Body/f_mk_ins.htm#make-instance"><B>MAKE-INSTANCE</B></A>, but all the others just use "&REST". This asymmetry is an<P>
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oversight due to the fact that the <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> combination code special cases them,<P>
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and elides normal generic function keyword checking for all of them (hence it<P>
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has never appeared as a "bug"). MCL (like Symbolics and Chestnut) also<P>
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provides the additional behavior that if one of the applicable methods<P>
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specifies <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> then the initarg checking for the protocol is not<P>
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performed.<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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Some possibly tricky code in the implementation may have to be reexamined. In<P>
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some implementations it looks like this may involve simply ripping out some<P>
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special cases.<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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Probably none in most cases. Since none of the implementations surveyed ever<P>
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signal an error for the described problem case, it is unlikely that any user<P>
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code exists which depends on an error being signaled.<P>
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<P>
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Users can eliminate unnecessary <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> from initialization methods<P>
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at their leisure, since its presence or absence has no effect.<P>
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<P>
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However, those users who have made use of the fairly common but non-portable<P>
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meaning of <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in applicable methods as a means of suppressing<P>
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initarg validation would have to find some other mechanism for this purpose.<P>
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One possibility would be to define appropriate :AROUND or primary methods on<P>
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<A REL=DEFINITION HREF="../Body/f_mk_ins.htm#make-instance"><B>MAKE-INSTANCE</B></A> that add :ALLOW-OTHER-KEYS T to the initialization arguments.<P>
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<P>
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<B>Performance Impact:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Users and implementors will know what <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in an initialization<P>
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<A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A>'s lambda-list means. Programmers writing initialization methods that<P>
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specify keywords won't have to remember to include <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A><P>
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explicitly. <P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Requiring all initialization methods that specify <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> to also specify<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> is clearly unaesthetic, so this change is an improvement.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Interestingly, for one reason or another, none of the implementations surveyed<P>
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perform generic function keyword checking (as opposed to initarg checking) for<P>
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the functions affected by this proposal.<P>
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<P>
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The possibility of having <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in the lambda-list of an<P>
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applicable initialization <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> mean that initarg validation should be<P>
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inhibited was discussed, especially in light of the fact that several of the<P>
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implementations surveyed already <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> this behavior. However, some people<P>
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questioned whether this was really such a good idea, being unmodular and<P>
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perhaps *too* convenient. Also, making such a change would add a real cost to<P>
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users who weren't expecting this extension, since they would have to examine<P>
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occurances of <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> in initialization methods to decide whether<P>
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they were left over from the time when it was (at least theoretically) needed<P>
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to inhibit generic function keyword checking or actually there to do the new<P>
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thing.<P>
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</PRE>
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<HR>
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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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