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155 lines
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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>4.8. Type Conversion Function</TITLE>
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<BODY>
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<meta name="description" value=" Type Conversion Function">
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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=tex2html2151 HREF="node53.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2149 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2143 HREF="node51.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2153 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2154 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2152 HREF="node53.html"> Determining the Type </A>
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<B>Up:</B> <A NAME=tex2html2150 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2144 HREF="node51.html"> Defining New Type </A>
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<HR> <P>
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<H1><A NAME=SECTION00880000000000000000>4.8. Type Conversion Function</A></H1>
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<P>
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The following function may be used to convert an object to an
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equivalent object of another type.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>coerce</tt> <tt><I>object</I></tt> <tt><I>result-type</I></tt><P>
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The <i>result-type</i> must be a type specifier; the <i>object</i> is converted
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to an ``equivalent'' object of the specified type.
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If the coercion cannot be performed, then an error is signaled.
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In particular, <tt>(coerce x 'nil)</tt> always signals an error.
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If <i>object</i> is already of the specified type, as determined
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by <tt>typep</tt>, then it is simply returned.
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It is not generally
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possible to convert any object to be of any type whatsoever; only certain
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conversions are permitted:
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<UL><LI>
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Any sequence type may be converted to any other sequence type, provided
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the new sequence can contain all actual elements of the old sequence
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(it is an error if it cannot). If the <i>result-type</i> is specified as
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simply <tt>array</tt>, for example, then <tt>(array t)</tt> is assumed. A
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specialized type such as <tt>string</tt> or <tt>(vector (complex short-float))</tt>
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may be specified; of course, the result may be of either that type or
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some more general type, as determined by the implementation.
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Elements of the new sequence will be <tt>eql</tt> to corresponding elements
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of the old sequence.
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If the
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<i>sequence</i> is already of the specified type, it may be returned without
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copying it; in this, <tt>(coerce <i>sequence</i> <i>type</i>)</tt> differs from
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<tt>(concatenate <i>type</i> <i>sequence</i>)</tt>, for the latter is required to
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copy the argument <i>sequence</i>. In particular, if one specifies
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<tt>sequence</tt>, then the argument may simply be returned if it already is
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a <tt>sequence</tt>.
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<P><pre>
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(coerce '(a b c) 'vector) => #(a b c)
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</pre><P>
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</UL>
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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 (SEQUENCE-TYPE-LENGTH) <A NAME=2678> </A> to specify that
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<tt>coerce</tt> should signal an error if the new sequence type specifies the
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number of elements and the old sequence has a different length.
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<P>
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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=2682> </A>
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to specify that if the <i>result-type</i> is <tt>string</tt>
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then it is understood to mean <tt>(vector character)</tt>,
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and <tt>simple-string</tt> is understood to mean <tt>(simple-array character (*))</tt>.
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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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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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<UL><LI>
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Some strings, symbols, and integers may be converted to characters.
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If <i>object</i> is a string of length 1, then the
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sole element of the string is returned. If <i>object</i> is a symbol
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whose print name is of length 1, then the sole element of the print name
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is returned. If <i>object</i> is an integer <i>n</i>, then <tt>(int-char <i>n</i>)</tt>
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is returned. See <tt>character</tt>.
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<P><pre>
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(coerce "a" 'character) => #\a
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</pre><P>
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</UL>
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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 (CHARACTER-PROPOSAL) <A NAME=2703> </A>
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to eliminate <tt>int-char</tt> from Common Lisp.
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Presumably this eliminates the possibility of coercing an
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integer to a character, although the vote did not address
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this question directly.
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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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<UL><LI>
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Any non-complex number can be converted to a <tt>short-float</tt>,
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<tt>single-float</tt>, <tt>double-float</tt>, or <tt>long-float</tt>. If simply <tt>float</tt>
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is specified, and <i>object</i> is not already a <tt>float</tt> of some kind, then
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the object is converted to a <tt>single-float</tt>.
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<P><pre>
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(coerce 0 'short-float) => 0.0S0
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(coerce 3.5L0 'float) => 3.5L0
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(coerce 7/2 'float) => 3.5
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</pre><P>
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<P>
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<LI>
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Any number can be converted to a complex number.
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If the number is not already complex, then a zero imaginary part
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is provided by coercing the integer zero to the type of the given real part.
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(If the given real part is rational, however, then the rule of
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canonical representation for complex rationals will result
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in the immediate re-conversion of the result from type <tt>complex</tt>
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back to type <tt>rational</tt>.)
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<P><pre>
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(coerce 4.5s0 'complex) => #C(4.5S0 0.0S0)
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(coerce 7/2 'complex) => 7/2
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(coerce #C(7/2 0) '(complex double-float))
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=> #C(3.5D0 0.0D0)
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</pre><P>
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<P>
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<LI>
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Any object may be coerced to type <tt>t</tt>.
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<P><pre>
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(coerce x 't) == (identity x) == x
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</pre><P>
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</UL>
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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 1988 (FUNCTION-TYPE) <A NAME=2726> </A>
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to allow coercion of certain objects to the type <tt>function</tt>:
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<UL><LI>
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A symbol or lambda-expression can be converted to a function.
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A symbol is coerced to type <tt>function</tt> as if by applying
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<tt>symbol-function</tt> to the symbol; an error is signaled if the predicate
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<tt>fboundp</tt> is not true of
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the symbol or if the symbol names a macro or special form.
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A list <i>x</i> whose <i>car</i> is the symbol <tt>lambda</tt>
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is coerced to a function as if by execution of <tt>(eval `#',<i>x</i>)</tt>,
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that is, of <tt>(eval (list 'function <i>x</i>))</tt>.
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</UL>
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<img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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Coercions from floating-point numbers to rationals and from ratios
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to integers are purposely <i>not</i> provided because of rounding
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problems. The functions <tt>rational</tt>, <tt>rationalize</tt>,
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<tt>floor</tt>, <tt>ceiling</tt>, <tt>truncate</tt>, and <tt>round</tt> may be used for
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such purposes. Similarly, coercions from characters to integers
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are purposely not provided; <tt>char-code</tt> or <tt>char-int</tt> may be
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used explicitly to perform such conversions.
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<P>
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<BR> <HR><A NAME=tex2html2151 HREF="node53.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2149 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2143 HREF="node51.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2153 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2154 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2152 HREF="node53.html"> Determining the Type </A>
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<B>Up:</B> <A NAME=tex2html2150 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2144 HREF="node51.html"> Defining New Type </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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