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54 lines
2.8 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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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>4.3. Predicating Type Specifiers</TITLE>
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html2091 HREF="node48.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2089 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2083 HREF="node46.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2093 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2094 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2092 HREF="node48.html"> Type Specifiers That </A>
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<B>Up:</B> <A NAME=tex2html2090 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2084 HREF="node46.html"> Type Specifier Lists</A>
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<HR> <P>
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<H1><A NAME=SECTION00830000000000000000>4.3. Predicating Type Specifiers</A></H1>
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<P>
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<A NAME=PREDICATINGTYPESPECIFIERSSECTION>A</A>
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type specifier list <tt>(satisfies <i>predicate-name</i>)</tt> denotes
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the set of all objects that satisfy the predicate named by <i>predicate-name</i>,
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which must be a symbol whose global function definition is a one-argument
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predicate.
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(A name is required; lambda-expressions are disallowed in order to avoid
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scoping problems.) For example, the type <tt>(satisfies numberp)</tt> is the
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same as the type <tt>number</tt>.
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The call <tt>(typep x '(satisfies p))</tt> results in applying <tt>p</tt> to <tt>x</tt>
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and returning <tt>t</tt> if the result is true and <tt>nil</tt> if the result is false.
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<P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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As an example, the type <tt>string-char</tt> could be defined as
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<P><pre>
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(deftype string-char ()
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'(and character (satisfies string-char-p)))
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</pre><P>
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See <tt>deftype</tt>.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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X3J13 voted in March 1989 (COMMON-TYPE) <A NAME=2151> </A> to remove the type <tt>string-char</tt>
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and the function <tt>string-char-p</tt> from the language.
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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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It is not a good idea for
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a predicate appearing in a <tt>satisfies</tt> type specifier to
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cause any side effects when invoked.
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<P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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