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91 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>19.7.2. Named Structures</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Named 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=tex2html3739 HREF="node178.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3737 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3731 HREF="node176.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3741 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3742 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3740 HREF="node178.html"> Other Aspects of </A>
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<B>Up:</B> <A NAME=tex2html3738 HREF="node175.html"> Structures of Explicitly </A>
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<B> Previous:</B> <A NAME=tex2html3732 HREF="node176.html"> Unnamed Structures</A>
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<HR> <P>
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<H2><A NAME=SECTION002372000000000000000>19.7.2. Named Structures</A></H2>
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<P>
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A ``named'' structure has the property that, given an instance of the
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structure, the structure name (that names the type) can be reliably
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recovered. For structures defined
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with no <tt>:type</tt> option, the structure name actually becomes part
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of the Common Lisp data-type system. The function <tt>type-of</tt>,
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when applied to such a structure, will return the structure name
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as the type of the object; the predicate <tt>typep</tt> will recognize
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the structure name as a valid type specifier.
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<P>
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For structures defined with a <tt>:type</tt> option, <tt>type-of</tt> will
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return a type specifier such as <tt>list</tt> or <tt>(vector t)</tt>,
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depending on the type specified to the <tt>:type</tt> option.
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The structure name does not become a valid type specifier.
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However,
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if the <tt>:named</tt> option is also specified, then the first component
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of the structure (as created by a <tt>defstruct</tt> constructor function)
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will always contain the structure name. This allows the structure name
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to be recovered from an instance of the structure and allows a reasonable
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predicate for the conceptual type to be defined:
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the automatically defined
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<tt><i>name</i>-p</tt> predicate for the structure operates by first
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checking that its argument is of the proper type (<tt>list</tt>, <tt>(vector t)</tt>,
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or whatever) and then checking whether the first component contains
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the appropriate type name.
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<P>
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Consider the <tt>binop</tt> example shown above, modified only to
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include the <tt>:named</tt> option:
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<P><pre>
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(defstruct (binop (:type list) :named)
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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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As before, this will define a constructor function <tt>make-binop</tt> and three
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selector functions <tt>binop-operator</tt>, <tt>binop-operand-1</tt>,
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and <tt>binop-operand-2</tt>. It will also define a predicate <tt>binop-p</tt>.
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<P>
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The effect of <tt>make-binop</tt> is now to construct a list of length 4:
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<P><pre>
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(make-binop :operator '+ :operand-1 'x :operand-2 5)
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=> (binop + x 5)
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(make-binop :operand-2 4 :operator '*)
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=> (binop * <tt>nil</tt> 4)
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</pre><P>
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The structure has the same layout as before except that the structure name
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<tt>binop</tt> is included as the first list element.
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The selector functions
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<tt>binop-operator</tt>, <tt>binop-operand-1</tt>,
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and <tt>binop-operand-2</tt> are essentially equivalent to <tt>cadr</tt>,
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<tt>caddr</tt>, and <tt>cadddr</tt>, respectively.
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The predicate <tt>binop-p</tt> is more or less equivalent to the following
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definition.
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<P><pre>
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(defun binop-p (x)
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(and (consp x) (eq (car x) 'binop)))
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</pre><P>
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The name <tt>binop</tt> is still not a valid type specifier recognizable
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to <tt>typep</tt>, but at least there is a way of distinguishing <tt>binop</tt>
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structures from other similarly defined structures.
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<P>
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<br><HR><A NAME=tex2html3739 HREF="node178.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3737 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3731 HREF="node176.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3741 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3742 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3740 HREF="node178.html"> Other Aspects of </A>
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<B>Up:</B> <A NAME=tex2html3738 HREF="node175.html"> Structures of Explicitly </A>
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<B> Previous:</B> <A NAME=tex2html3732 HREF="node176.html"> Unnamed 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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