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110 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>7.7. Blocks and Exits</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Blocks and Exits">
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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=tex2html2584 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2582 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2576 HREF="node84.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2586 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2587 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2585 HREF="node86.html"> Iteration</A>
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<B>Up:</B> <A NAME=tex2html2583 HREF="node76.html"> Control Structure</A>
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<B> Previous:</B> <A NAME=tex2html2577 HREF="node84.html"> Conditionals</A>
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<HR> <P>
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<H1><A NAME=SECTION001170000000000000000>7.7. Blocks and Exits</A></H1>
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<P>
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<A NAME=BLOCKRETURNSECTION>The</A>
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<tt>block</tt> and <tt>return-from</tt> constructs provide a structured lexical
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non-local exit facility. At any point lexically within a <tt>block</tt>
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construct, a <tt>return-from</tt> with the same name may be used to
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perform an immediate transfer of control that
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exits from the <tt>block</tt>. In the most common cases this mechanism is
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more efficient than the dynamic non-local exit facility
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provided by <tt>catch</tt> and <tt>throw</tt>, described in
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section <A HREF="node96.html#CATCHTHROWSECTION">7.11</A>.
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<P>
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<BR><b>[Special Form]</b><BR>
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<tt>block <i>name</i> {<i>form</i>}*</tt><P>The <tt>block</tt> construct executes each <i>form</i> from left to right,
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returning whatever is returned by the last <i>form</i>.
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If, however, a <tt>return</tt> or <tt>return-from</tt> form that specifies the
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same <i>name</i> is executed
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during the execution of some <i>form</i>, then the results
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specified by the <tt>return</tt> or <tt>return-from</tt> are immediately
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returned as the value of the <tt>block</tt> construct, and execution
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proceeds as if the <tt>block</tt> had terminated normally.
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In this, <tt>block</tt> differs from <tt>progn</tt>; the <tt>progn</tt> construct
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has nothing to do with <tt>return</tt>.
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<P>
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The <i>name</i> is not evaluated; it must be a symbol.
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The scope of the <i>name</i> is lexical; only a <tt>return</tt> or <tt>return-from</tt>
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textually contained in some <i>form</i> can exit from the block.
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The extent of the name is dynamic.
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Therefore it is only possible to exit from a given run-time incarnation of a
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block once, either normally or by explicit return.
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<P>
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The <tt>defun</tt> form implicitly puts a <tt>block</tt> around the
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body of the function defined; the <tt>block</tt> has the same name as the function.
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Therefore one may use <tt>return-from</tt> to return
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prematurely from a function defined by <tt>defun</tt>.
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<P>
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The lexical scoping of the block name
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is fully general and has consequences that may be surprising
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to users and implementors of other Lisp systems.
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For example, the <tt>return-from</tt> in the following example actually does
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work in Common Lisp as one might expect:
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<P><pre>
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(block loser
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(catch 'stuff
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(mapcar #'(lambda (x) (if (numberp x)
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(hairyfun x)
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(return-from loser <tt>nil</tt>)))
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items)))
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</pre><P>
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Depending on the situation, a <tt>return</tt> in Common Lisp
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may not be simple.
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A <tt>return</tt> can break up catchers if necessary to get
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to the block in question.
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It is possible for a ``closure'' created by <tt>function</tt> for
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a lambda-expression to refer to a block name as long as the name
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is lexically apparent.
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<P>
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<BR><b>[Special Form]</b><BR>
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<tt>return-from <i>name</i> [<i>result</i>]</tt><P><tt>return-from</tt>
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is used to return from a <tt>block</tt> or from such constructs
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as <tt>do</tt> and <tt>prog</tt> that implicitly establish a <tt>block</tt>.
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The <i>name</i> is not evaluated and must be a symbol.
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A <tt>block</tt> construct with the same name must lexically
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enclose the occurrence of <tt>return-from</tt>;
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whatever the evaluation of <i>result</i> produces
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is immediately returned from the block.
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(If the <i>result</i> form is omitted, it defaults to <tt>nil</tt>.
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As a matter of style, this form ought to be used to indicate that
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the particular value returned doesn't matter.)
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<P>
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The <tt>return-from</tt> form itself never returns and cannot have a value;
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it causes results to be returned from a <tt>block</tt> construct.
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If the evaluation of <i>result</i> produces multiple values,
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those multiple values are returned by the construct exited.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>return</tt> <tt>[<i>result</i>]</tt><P><tt>(return <i>form</i>)</tt> is identical in meaning
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to <tt>(return-from <tt>nil</tt> <i>form</i>)</tt>; it returns from a block named <tt>nil</tt>.
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Blocks established implicitly by iteration constructs such
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as <tt>do</tt> are named <tt>nil</tt>, so that <tt>return</tt> will exit properly from
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such a construct.
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<P>
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<BR> <HR><A NAME=tex2html2584 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2582 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2576 HREF="node84.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2586 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2587 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2585 HREF="node86.html"> Iteration</A>
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<B>Up:</B> <A NAME=tex2html2583 HREF="node76.html"> Control Structure</A>
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<B> Previous:</B> <A NAME=tex2html2577 HREF="node84.html"> Conditionals</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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