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138 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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<TITLE>20.2. The Top-Level Loop</TITLE>
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<meta name="description" value=" The Top-Level Loop">
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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=tex2html3785 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3783 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3779 HREF="node180.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3787 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3788 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3786 HREF="node182.html"> Streams</A>
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<B>Up:</B> <A NAME=tex2html3784 HREF="node179.html"> The Evaluator</A>
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<B> Previous:</B> <A NAME=tex2html3780 HREF="node180.html"> Run-Time Evaluation of </A>
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<HR> <P>
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<H1><A NAME=SECTION002420000000000000000>20.2. The Top-Level Loop</A></H1>
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<P>
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Normally one interacts with Lisp through a ``top-level
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<tt>read</tt>-<tt>eval</tt>-<tt>print</tt> loop,'' so called because
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it is the highest level of control and consists of an endless
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loop that reads an expression, evaluates it, and prints the
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results. One has an effect on the state of the Lisp system
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only by invoking actions that have side effects.
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<P>
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The precise nature of the top-level loop for Common Lisp
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is purposely not rigorously specified here so that implementors can
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experiment to improve the user interface.
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For example, an implementor may choose to require line-at-a-time
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input, or may provide a fancy editor or complex graphics-display
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interface. An implementor may choose to provide
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explicit prompts for input,
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or may choose (as MacLisp does) not to clutter up the transcript
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with prompts.
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<P>
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The top-level loop is required to trap all throws and recover gracefully.
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It is also required to print all values resulting from evaluation of a form,
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perhaps on separate lines. If a form returns zero values, as little
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as possible should be printed.
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<P>
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The following variables are maintained by the top-level loop
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as a limited safety net, in case the user forgets to save an interesting input
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expression or output value. (Note that the names of some of these variables
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violate the convention that names of global variables begin and end with
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an asterisk.) These are intended primarily for user interaction, which is why
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they have short names. Use of these variables should be avoided in programs.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>+</tt> <tt><BR></tt><tt>++</tt> <tt><BR></tt><tt>+++</tt><P>While a form is being evaluated by the top-level loop,
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the variable <tt>+</tt> is bound to the previous form read by the loop.
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The variable <tt>++</tt> holds the previous value of <tt>+</tt> (that is, the form
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evaluated two interactions ago), and <tt>+++</tt> holds the previous value
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of <tt>++</tt>.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>-</tt><P>While a form is being evaluated by the top-level loop,
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the variable <tt>-</tt> is bound to the form itself; that is,
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it is the value about to be given to <tt>+</tt> once this interaction
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is done.
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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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<i>Notice of correction.</i>
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In the first edition, the name of the variable <tt>-</tt> was
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inadvertently omitted.
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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>[Variable]</b><BR>
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<tt>*<BR></tt><tt>**</tt> <tt><BR></tt><tt>***</tt><P>While a form is being evaluated by the top-level loop,
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the variable <tt>*</tt> is bound to the result printed at the
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end of the last time through the loop; that is, it is the value
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produced by evaluating the form in <tt>+</tt>. If several values were produced,
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<tt>*</tt> contains the first value only; <tt>*</tt> contains <tt>nil</tt> if zero values
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were produced.
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The variable <tt>**</tt> holds the previous value of <tt>*</tt> (that is, the result
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printed two interactions ago), and <tt>***</tt> holds the previous value
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of <tt>**</tt>.
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<P>
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If the evaluation of <tt>+</tt> is aborted for some reason,
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then the values associated with <tt>*</tt>, <tt>**</tt>, and <tt>***</tt> are not updated;
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they are updated only if the printing of values is at least begun (though not
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necessarily completed).
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>/</tt> <tt><BR></tt><tt>//</tt> <tt><BR></tt><tt>///</tt><P>While a form is being evaluated by the top-level loop,
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the variable <tt>/</tt> is bound to a list of the results printed at the
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end of the last time through the loop; that is, it is a list of all values
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produced by evaluating the form in <tt>+</tt>. The value of <tt>*</tt> should
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always be the same as the <i>car</i> of the value of <tt>/</tt>.
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The variable <tt>//</tt> holds the previous value of <tt>/</tt> (that is, the results
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printed two interactions ago), and <tt>///</tt> holds the previous value
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of <tt>//</tt>. Therefore the value of <tt>**</tt> should always be the same
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as the <i>car</i> of <tt>//</tt>, and similarly for <tt>***</tt> and <tt>///</tt>.
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<P>
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If the evaluation of <tt>+</tt> is aborted for some reason,
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then the values associated with <tt>/</tt>, <tt>//</tt>, and <tt>///</tt> are not updated;
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they are updated only if the printing of values is at least begun (though not
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necessarily completed).
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<P>
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As an example of the processing of these variables, consider the
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following possible transcript, where <tt>></tt> is a prompt by
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the top-level loop for user input:
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<P><pre>
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>(cons - -) ;Interaction 1
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((CONS - -) CONS - -) ;Cute, huh?
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>(values) ;Interaction 2
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;Nothing to print
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>(cons 'a 'b) ;Interaction 3
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(A . B) ;There is a single value
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>(hairy-loop)^G ;Interaction 4
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### QUIT to top level. ;(User aborts the computation.)
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>(floor 13 4) ;Interaction 5
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3 ;There are two values
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1
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</pre><P>
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At this point we have:
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<P><pre>
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+++ => (cons 'a 'b) *** => NIL /// => ()
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++ => (hairy-loop) ** => (A . B) // => ((A . B))
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+ => (floor 13 4) * => 3 / => (3 1)
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</pre><P>
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<P>
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<BR> <HR><A NAME=tex2html3785 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3783 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3779 HREF="node180.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3787 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3788 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3786 HREF="node182.html"> Streams</A>
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<B>Up:</B> <A NAME=tex2html3784 HREF="node179.html"> The Evaluator</A>
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<B> Previous:</B> <A NAME=tex2html3780 HREF="node180.html"> Run-Time Evaluation of </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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