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4.7 KiB
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99 lines
4.7 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>19.7.3. Other Aspects of Explicitly Specified Structures</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Other Aspects of Explicitly Specified Structures">
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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=tex2html3749 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3747 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3743 HREF="node177.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3751 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3752 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3750 HREF="node179.html"> The Evaluator</A>
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<B>Up:</B> <A NAME=tex2html3748 HREF="node175.html"> Structures of Explicitly </A>
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<B> Previous:</B> <A NAME=tex2html3744 HREF="node177.html"> Named Structures</A>
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<HR> <P>
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<H2><A NAME=SECTION002373000000000000000>19.7.3. Other Aspects of Explicitly Specified Structures</A></H2>
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<P>
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<A NAME=DEFSTRUCTINITIALOFFSET>The</A>
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<tt>:initial-offset</tt> option allows one
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to specify that slots be allocated beginning at a representational
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element other than the first. For example, the form
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<P><pre>
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(defstruct (binop (:type list) (:initial-offset 2))
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(operator '? :type symbol)
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operand-1
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operand-2)
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</pre><P>
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would result in the following behavior for <tt>make-binop</tt>:
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<P><pre>
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(make-binop :operator '+ :operand-1 'x :operand-2 5)
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=> (nil nil + x 5) <BR><BR>(make-binop :operand-2 4 :operator '*)
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=> (nil nil * <tt>nil</tt> 4)
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</pre><P>
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The selectors
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<tt>binop-operator</tt>, <tt>binop-operand-1</tt>,
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and <tt>binop-operand-2</tt> would be essentially equivalent to <tt>caddr</tt>,
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<tt>cadddr</tt>, and <tt>car</tt> of <tt>cddddr</tt>, respectively.
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Similarly, the form
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<P><pre>
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(defstruct (binop (:type list) :named (:initial-offset 2))
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(operator '? :type symbol)
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operand-1
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operand-2)
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</pre><P>
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would result in the following behavior for <tt>make-binop</tt>:
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<P><pre>
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(make-binop :operator '+ :operand-1 'x :operand-2 5)
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=> (nil nil binop + x 5)
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(make-binop :operand-2 4 :operator '*)
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=> (nil nil binop * <tt>nil</tt> 4)
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</pre><P>
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<P>
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If the <tt>:include</tt> is used with the <tt>:type</tt>
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option, then the effect is first to skip over as many representation
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elements as needed to represent the included structure, then to
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skip over any additional elements specified by the <tt>:initial-offset</tt>
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option, and then to begin allocation of elements from that point.
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For example:
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<P><pre>
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(defstruct (binop (:type list) :named (:initial-offset 2))
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(operator '? :type symbol)
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operand-1
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operand-2)
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(defstruct (annotated-binop (:type list)
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(:initial-offset 3)
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(:include binop))
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commutative associative identity)
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(make-annotated-binop :operator '*
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:operand-1 'x
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:operand-2 5
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:commutative t
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:associative t
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:identity 1)
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=> (nil nil binop * x 5 nil nil nil t t 1)
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</pre><P>
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The first two <tt>nil</tt> elements stem from the <tt>:initial-offset</tt> of <tt>2</tt>
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in the definition of <tt>binop</tt>. The next four elements contain the
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structure name and three slots for <tt>binop</tt>. The next three <tt>nil</tt> elements
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stem from the <tt>:initial-offset</tt> of <tt>3</tt> in the definition of
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<tt>annotated-binop</tt>. The last three list elements contain the additional
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slots for an <tt>annotated-binop</tt>.
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<P>
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<BR> <HR><A NAME=tex2html3749 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3747 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3743 HREF="node177.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3751 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3752 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3750 HREF="node179.html"> The Evaluator</A>
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<B>Up:</B> <A NAME=tex2html3748 HREF="node175.html"> Structures of Explicitly </A>
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<B> Previous:</B> <A NAME=tex2html3744 HREF="node177.html"> Named Structures</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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