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168 lines
8.1 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>7.1.2. Assignment</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Assignment">
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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=tex2html2512 HREF="node80.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2510 HREF="node77.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2506 HREF="node78.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2514 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2515 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2513 HREF="node80.html"> Generalized Variables</A>
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<B>Up:</B> <A NAME=tex2html2511 HREF="node77.html"> Constants and Variables</A>
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<B> Previous:</B> <A NAME=tex2html2507 HREF="node78.html"> Reference</A>
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<HR> <P>
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<H2><A NAME=SECTION001112000000000000000>7.1.2. Assignment</A></H2>
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<P>
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The following facilities allow the value of a variable (more specifically,
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the value associated with the current binding of the variable) to be altered.
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Such alteration is different from establishing a new binding.
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Constructs for establishing new bindings of variables are described
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in section <A HREF="node83.html#VARBINDINGSECTION">7.5</A>.
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<P>
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<BR><b>[Special Form]</b><BR>
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<tt>setq</tt> <tt>{<i>var</I></tt> <tt><i>form</I>}*</tt><P>The special form <tt>(setq <i>var1</i> <i>form1</i> <i>var2</i> <i>form2</i> ...)</tt> is the
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``simple variable assignment statement'' of Lisp.
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First <i>form1</i> is evaluated
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and the result is stored in the variable <i>var1</i>, then <i>form2</i>
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is evaluated and the result stored in <i>var2</i>, and so forth.
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The variables are represented as symbols, of course, and are interpreted
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as referring to static or dynamic instances according to the usual rules.
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Therefore <tt>setq</tt> may be used for assignment of both lexical
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and special variables.
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<P>
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<tt>setq</tt> returns the last value assigned, that is, the result of the
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evaluation of its last argument.
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As a boundary case, the form <tt>(setq)</tt> is legal and returns <tt>nil</tt>.
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There must be an even number of argument forms.
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For example, in
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<P><pre>
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(setq x (+ 3 2 1) y (cons x nil))
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</pre><P>
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<tt>x</tt> is set to <tt>6</tt>, <tt>y</tt> is set to <tt>(6)</tt>, and the <tt>setq</tt>
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returns <tt>(6)</tt>. Note that the first assignment is performed before
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the second form is evaluated, allowing that form to
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use the new value of <tt>x</tt>.
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<P>
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See also the description of <tt>setf</tt>,
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the Common Lisp ``general assignment statement'' that is capable of assigning
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to variables, array elements, and other locations.
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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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Some programmers choose to avoid
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<tt>setq</tt> as a matter of style, always using <tt>setf</tt> for any kind of
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structure modification. Others use <tt>setq</tt> with simple variable names and
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<tt>setf</tt> with all other generalized variables.
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<P>
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X3J13 voted in March 1989
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(SYMBOL-MACROLET-SEMANTICS) <A NAME=5070> </A> to specify that if any <i>var</i>
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refers not to an ordinary variable but to a binding made by
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<tt>symbol-macrolet</tt>, then that <i>var</i> is handled as
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if <tt>setf</tt> had been used instead of <tt>setq</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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<BR><b>[Macro]</b><BR>
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<tt>psetq</tt> <tt>{<i>var</I></tt> <tt><i>form</I>}*</tt><P>A <tt>psetq</tt> form is just like a <tt>setq</tt> form, except
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that the assignments happen in parallel. First all of the forms
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are evaluated, and then the variables are set to the resulting
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values. The value of the <tt>psetq</tt> form is <tt>nil</tt>.
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For example:
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<P><pre>
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(setq a 1)
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(setq b 2)
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(psetq a b b a)
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a => 2
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b => 1
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</pre><P>
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In this example, the values of <tt>a</tt> and <tt>b</tt> are exchanged by
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using parallel assignment.
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(If several variables are to be assigned in parallel in
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the context of a loop, the <tt>do</tt> construct may be appropriate.)
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<P>
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See also the description of <tt>psetf</tt>,
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the Common Lisp ``general parallel assignment statement'' that
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is capable of assigning
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to variables, array elements, and other locations.
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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
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(SYMBOL-MACROLET-SEMANTICS) <A NAME=5091> </A> to specify that if any <i>var</i>
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refers not to an ordinary variable but to a binding made by
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<tt>symbol-macrolet</tt>, then that <i>var</i> is handled as
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if <tt>psetf</tt> had been used instead of <tt>psetq</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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<BR><b>[Function]</b><BR>
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<tt>set</tt> <tt><i>symbol</I></tt> <tt><i>value</I></tt><P><tt>set</tt> allows alteration of the value of a dynamic (special) variable.
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<tt>set</tt> causes the dynamic variable named by <i>symbol</i> to take on
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<i>value</i> as its value.
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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 January 1989
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(ARGUMENTS-UNDERSPECIFIED) <A NAME=5105> </A>
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to clarify that the <i>value</i>
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may be any Lisp datum whatsoever.
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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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Only the value of the current dynamic binding is altered;
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if there are no bindings in effect, the most global value is altered.
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For example,
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<P><pre>
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(set (if (eq a b) 'c 'd) 'foo)
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</pre><P>
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will either set <tt>c</tt> to <tt>foo</tt> or set <tt>d</tt> to <tt>foo</tt>, depending
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on the outcome of the test <tt>(eq a b)</tt>.
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<P>
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<tt>set</tt> returns <i>value</i> as its result.
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<P>
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<tt>set</tt> cannot alter
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the value of a local (lexically bound) variable.
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The special form <tt>setq</tt>
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is usually used for altering the values of variables
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(lexical or dynamic) in programs.
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<tt>set</tt> is particularly useful for implementing interpreters for
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languages embedded in Lisp.
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See also <tt>progv</tt>, a construct that performs binding rather
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than assignment of dynamic variables.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>makunbound</tt> <tt><i>symbol</I></tt> <tt><br></tt><tt>fmakunbound</tt> <tt><i>symbol</I></tt><P><tt>makunbound</tt> causes the dynamic (special) variable named
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by <i>symbol</i> to become unbound (have no value). <tt>fmakunbound</tt>
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does the analogous thing for the global function definition named
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by <i>symbol</i>.
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For example:
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<P><pre>
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(setq a 1)
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a => 1
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(makunbound 'a)
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a => causes an error
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(defun foo (x) (+ x 1))
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(foo 4) => 5
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(fmakunbound 'foo)
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(foo 4) => causes an error
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</pre><P>
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Both functions return <i>symbol</i> as the result value.
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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 (FUNCTION-NAME) <A NAME=5133> </A> to extend <tt>fmakunbound</tt>
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to accept any function-name (a symbol or a list
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whose <i>car</i> is <tt>setf</tt> - see section <A HREF="node77.html#FUNCTIONNAMESECTION">7.1</A>).
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Thus one may write <tt>(fmakunbound '(setf cadr))</tt> to remove any
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global definition of a <tt>setf</tt> expansion function for <tt>cadr</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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<BR> <HR><A NAME=tex2html2512 HREF="node80.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2510 HREF="node77.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2506 HREF="node78.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2514 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2515 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2513 HREF="node80.html"> Generalized Variables</A>
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<B>Up:</B> <A NAME=tex2html2511 HREF="node77.html"> Constants and Variables</A>
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<B> Previous:</B> <A NAME=tex2html2507 HREF="node78.html"> Reference</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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</BODY>
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