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89 lines
4.9 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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<HEAD>
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<TITLE>9.3. Type Declaration for Forms</TITLE>
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<BODY>
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<meta name="description" value=" Type Declaration for Forms">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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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=tex2html2841 HREF="node107.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2839 HREF="node103.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2835 HREF="node105.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2843 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2844 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2842 HREF="node107.html"> Symbols</A>
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<B>Up:</B> <A NAME=tex2html2840 HREF="node103.html"> Declarations</A>
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<B> Previous:</B> <A NAME=tex2html2836 HREF="node105.html"> Declaration Specifiers</A>
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<HR> <P>
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<H1><A NAME=SECTION001330000000000000000>9.3. Type Declaration for Forms</A></H1>
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<P>
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Frequently it is useful to declare that the value produced
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by the evaluation of some form will be of a particular type.
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Using <tt>declare</tt> one can declare the type of the value
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held by a bound variable, but there is no easy way to declare
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the type of the value of an unnamed form. For this purpose the <tt>the</tt>
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special form is defined; <tt>(the <i>type</i> <i>form</i>)</tt> means
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that the value of <i>form</i> is declared to be of type <i>type</i>.
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<P>
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<BR><b>[Special Form]</b><BR>
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<tt>the</tt> <tt><i>value-type</i></tt> <tt><i>form</i></tt><P>The <i>form</i> is evaluated; whatever it produces is returned by
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the <tt>the</tt> form. In addition, it is an error if what is produced
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by the <i>form</i> does not conform to the data type specified by <i>value-type</i>
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(which is not evaluated). (A given implementation may or may not
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actually check for this error. Implementations are encouraged to make an
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explicit error check when running interpretively.) In effect, this
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declares that the user undertakes to guarantee that the values of
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the form will always be of the specified type.
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For example:
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<P><pre>
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(the string (copy-seq x)) ;The result will be a string
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(the integer (+ x 3)) ;The result of <tt>+</tt> will be an integer
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(+ (the integer x) 3) ;The value of <tt>x</tt> will be an integer
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(the (complex rational) (* z 3))
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(the (unsigned-byte 8) (logand x mask))
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</pre><P>
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The <tt>values</tt> type specifier may be used to indicate the types
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of multiple values:
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<P><pre>
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(the (values integer integer) (floor x y))
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(the (values string t)
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(gethash the-key the-string-table))
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</pre><P>
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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 June 1989 (THE-AMBIGUITY) <A NAME=9713> </A>
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to clarify that <i>value-type</i> may be any valid type specifier whatsoever.
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The point is that a type specifier need not be one suitable for
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discrimination but only for declaration.
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<P>
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In the case that the <i>form</i> produces exactly one value and <i>value-type</i>
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is not a <tt>values</tt> type specifier, one may describe a <tt>the</tt> form
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as being entirely equivalent to
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<P><pre>
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(let ((#1=#:temp <i>form</i>)) (declare (type <i>value-type</i> #1#)) #1#)
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</pre><P>
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A more elaborate expression could be written to describe the case where
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<i>value-type</i> is a <tt>values</tt> type specifier.
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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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<hr>
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<b>Compatibility note:</b> This construct is borrowed from the Interlisp DECL
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package; Interlisp, however, allows an implicit <tt>progn</tt> after the type
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specifier rather than just a single form. The MacLisp <tt>fixnum-identity</tt>
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and <tt>flonum-identity</tt> constructs can be expressed as <tt>(the fixnum <i>x</i>)</tt>
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and <tt>(the single-float <i>x</i>)</tt>.
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<hr>
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<P>
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<P>
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<BR> <HR><A NAME=tex2html2841 HREF="node107.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2839 HREF="node103.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2835 HREF="node105.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2843 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2844 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2842 HREF="node107.html"> Symbols</A>
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<B>Up:</B> <A NAME=tex2html2840 HREF="node103.html"> Declarations</A>
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<B> Previous:</B> <A NAME=tex2html2836 HREF="node105.html"> Declaration Specifiers</A>
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<HR> <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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