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91 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>28.1.6.5. Keyword Arguments in Generic Functions and Methods</TITLE>
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<meta name="description" value=" Keyword Arguments in Generic Functions and Methods">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5181 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5179 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5175 HREF="node281.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5183 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5184 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5182 HREF="node283.html"> Method Selection and </A>
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<B>Up:</B> <A NAME=tex2html5180 HREF="node277.html"> Generic Functions and </A>
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<B> Previous:</B> <A NAME=tex2html5176 HREF="node281.html"> Congruent Lambda-Lists for </A>
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<HR> <P>
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<H3><A NAME=SECTION003216500000000000000>28.1.6.5. Keyword Arguments in Generic Functions and Methods</A></H3>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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When a generic function or any of its methods mentions <tt>&key</tt> in
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a lambda-list, the specific set of keyword arguments accepted by the
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generic function varies according to the applicable methods. The set
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of keyword arguments accepted by the generic function for a particular
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call is the union of the keyword arguments accepted by all applicable
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methods and the keyword arguments mentioned after <tt>&key</tt> in the
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generic function definition, if any. A method that has <tt>&rest</tt>
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but not <tt>&key</tt> does not affect the set of acceptable keyword
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arguments. If the lambda-list of any applicable method or of the
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generic function definition contains <tt>&allow-other-keys</tt>, all
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keyword arguments are accepted by the generic function.
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<P>
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The lambda-list congruence rules require that each method
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accept all of the keyword arguments mentioned after <tt>&key</tt> in the
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generic function definition, by accepting them explicitly, by
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specifying <tt>&allow-other-keys</tt>, or by specifying <tt>&rest</tt> but
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not <tt>&key</tt>. Each method can accept additional keyword arguments
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of its own, in addition to the keyword arguments mentioned in the
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generic function definition.
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<P>
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<P>
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If a generic function is passed a keyword argument that no applicable
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method accepts, an error is signaled.
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<P>
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For example, suppose there are two methods defined for <tt>width</tt>
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as follows:
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<P>
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<P><pre>
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(defmethod width ((c character-class) &key font) ...)
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(defmethod width ((p picture-class) &key pixel-size) ...)
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</pre><P>
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<P>
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Assume that there are no other methods and no generic
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function definition for <tt>width</tt>. The evaluation of the
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following form will signal an error because the keyword argument
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<tt>:pixel-size</tt> is not accepted by the applicable method.
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<P>
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<P><pre>
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(width (make-instance 'character-class :char #\Q)
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:font 'baskerville :pixel-size 10)
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</pre><P>
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<P>
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The evaluation of the following form will signal an error.
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<P>
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<P><pre>
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(width (make-instance 'picture-class :glyph (glyph #\Q))
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:font 'baskerville :pixel-size 10)
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</pre><P>
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<P>
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The evaluation of the following form will not signal an error
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if the class named <tt>character-picture-class</tt> is a subclass of
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both <tt>picture-class</tt> and <tt>character-class</tt>.
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<P>
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<P><pre>
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(width (make-instance 'character-picture-class :char #\Q)
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:font 'baskerville :pixel-size 10)
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</pre><P>
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html5181 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5179 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5175 HREF="node281.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5183 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5184 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5182 HREF="node283.html"> Method Selection and </A>
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<B>Up:</B> <A NAME=tex2html5180 HREF="node277.html"> Generic Functions and </A>
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<B> Previous:</B> <A NAME=tex2html5176 HREF="node281.html"> Congruent Lambda-Lists for </A>
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<HR> <P>
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