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107 lines
5.7 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>7.4. Simple Sequencing</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Simple Sequencing">
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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=tex2html2548 HREF="node83.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2546 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2540 HREF="node81.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2550 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2551 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2549 HREF="node83.html"> Establishing New Variable </A>
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<B>Up:</B> <A NAME=tex2html2547 HREF="node76.html"> Control Structure</A>
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<B> Previous:</B> <A NAME=tex2html2541 HREF="node81.html"> Function Invocation</A>
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<HR> <P>
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<H1><A NAME=SECTION001140000000000000000>7.4. Simple Sequencing</A></H1>
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<P>
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Each of the constructs in this section simply evaluates all the
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argument forms in order. They differ only in what results
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are returned.
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<P>
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<BR><b>[Special Form]</b><BR>
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<tt>progn</tt> <tt>{<i>form</i>}*</tt><P>The <tt>progn</tt> construct takes a number of forms and evaluates
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them sequentially, in order, from left to right. The values of all
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the forms but the last are discarded; whatever the last form returns
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is returned by the <tt>progn</tt> form.
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One says that all the forms but the last are evaluated for <i>effect</i>,
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because their execution is useful only for the side effects caused,
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but the last form is executed for <i>value</i>.
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<P>
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<tt>progn</tt> is the primitive control structure construct for ``compound
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statements,'' such as <b>begin</b>-<b>end</b> blocks in
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Algol-like languages.
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Many Lisp constructs are ``implicit <tt>progn</tt>'' forms:
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as part of their syntax each allows many forms to be written
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that are to be evaluated sequentially, discarding the results
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of all forms but the last and returning the results of the last form.
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<P>
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If the last form of the <tt>progn</tt> returns multiple values, then those
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multiple values are returned by the <tt>progn</tt> form. If there are no forms
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for the <tt>progn</tt>, then the result is <tt>nil</tt>. These rules generally hold for
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implicit <tt>progn</tt> forms as well.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>prog1</tt> <tt><i>first</i></tt> <tt>{<i>form</i>}*</tt><P><tt>prog1</tt> is similar to <tt>progn</tt>, but it returns the value of
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its <i>first</i> form. All the argument forms are executed sequentially;
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the value of the first form is saved while all the others are executed
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and is then returned.
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<P>
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<tt>prog1</tt> is most commonly used to evaluate an expression with side
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effects and to return a value that must be computed <i>before</i> the side
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effects happen.
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For example:
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<P><pre>
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(prog1 (car x) (rplaca x 'foo))
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</pre><P>
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alters the <i>car</i> of <tt>x</tt> to be <tt>foo</tt> and returns the old <i>car</i>
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of <tt>x</tt>.
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<P>
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<tt>prog1</tt> always returns a single value, even if the first form
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tries to return multiple values.
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As a consequence,
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<tt>(prog1 <i>x</i>)</tt> and <tt>(progn <i>x</i>)</tt> may behave differently
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if <i>x</i> can produce multiple values. See <tt>multiple-value-prog1</tt>.
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A point of style:
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although <tt>prog1</tt> can be used to force exactly a single value to
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be returned, it is conventional to use the
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function <tt>values</tt> for this purpose.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>prog2</tt> <tt><i>first</i></tt> <tt><i>second</i></tt> <tt>{<i>form</i>}*</tt>
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<P><tt>prog2</tt> is similar to <tt>prog1</tt>, but it returns the value of
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its <i>second</i> form. All the argument forms are executed sequentially;
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the value of the second form
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is saved while all the other forms are executed and is then returned.
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<tt>prog2</tt> is provided mostly for historical compatibility.
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<P><pre>
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(prog2 <i>a</i> <i>b</i> <i>c</i> ... <i>z</i>) == (progn <i>a</i> (prog1 <i>b</i> <i>c</i> ... <i>z</i>))
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</pre><P>
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Occasionally it is desirable to perform one side effect, then a value-producing
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operation, then another side effect. In such a peculiar case, <tt>prog2</tt>
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is fairly perspicuous.
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For example:
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<P><pre>
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(prog2 (open-a-file) (process-the-file) (close-the-file))
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;value is that of <tt>process-the-file</tt>
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</pre><P>
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<P>
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<tt>prog2</tt>, like <tt>prog1</tt>,
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always returns a single value, even if the second form
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tries to return multiple values. As a consequence of this,
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<tt>(prog2 <i>x</i> <i>y</i>)</tt> and <tt>(progn <i>x</i> <i>y</i>)</tt> may behave differently
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if <i>y</i> can produce multiple values.
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<P>
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<BR> <HR><A NAME=tex2html2548 HREF="node83.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2546 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2540 HREF="node81.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2550 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2551 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2549 HREF="node83.html"> Establishing New Variable </A>
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<B>Up:</B> <A NAME=tex2html2547 HREF="node76.html"> Control Structure</A>
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<B> Previous:</B> <A NAME=tex2html2541 HREF="node81.html"> Function Invocation</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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