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85 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>4.10. Type Upgrading</TITLE>
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<BODY>
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<meta name="description" value=" Type Upgrading">
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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=tex2html2173 HREF="node55.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2171 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2167 HREF="node53.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2175 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2176 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2174 HREF="node55.html"> Program Structure</A>
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<B>Up:</B> <A NAME=tex2html2172 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2168 HREF="node53.html"> Determining the Type </A>
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<HR> <P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<H1><A NAME=SECTION008100000000000000000>4.10. Type Upgrading</A></H1>
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<P>
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X3J13 voted in January 1989
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(ARRAY-TYPE-ELEMENT-TYPE-SEMANTICS) <A NAME=2850> </A>
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to add new functions by which a program
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can determine, in a given Common Lisp implementation, how that
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implementation will <i>upgrade</i> a type when constructing an array
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specialized to contain elements of that type,
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or a complex number specialized to contain parts of that type.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>upgraded-array-element-type</tt> <tt><i>type</I></tt><P>
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A type specifier is returned, indicating the element type
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of the most specialized array representation capable of holding
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items of the specified argument <i>type</i>.
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The result is necessarily a supertype of the given <i>type</i>.
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Furthermore, if a type <i>A</i> is a subtype of type <i>B</i>, then
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<tt>(upgraded-array-element-type <i>A</i>)</tt> is a subtype of
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<tt>(upgraded-array-element-type <i>B</i>)</tt>.
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<P>
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The manner in which an array element type is upgraded depends
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only on the element type as such and not on any other property of
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the array such as size, rank, adjustability,
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presence or absence of a fill pointer, or displacement.
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<P>
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<hr>
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<b>Rationale:</b> If upgrading were allowed to depend on any of these properties,
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all of which can be referred to, directly or indirectly, in the
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language of type specifiers, then it would not be possible
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to displace an array in a consistent and dependable manner
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to another array created with the same <tt>:element-type</tt> argument
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but differing in one of these properties.
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<hr>
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<P>
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Note that <tt>upgraded-array-element-type</tt> could be defined as
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<P><pre>
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(defun upgraded-array-element-type (type)
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(array-element-type (make-array 0 :element-type type)))
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</pre><P>
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but this definition has the disadvantage of allocating an array and
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then immediately discarding it. The clever implementor surely can
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conjure up a more practical approach.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>upgraded-complex-part-type</tt> <tt><i>type</i></tt><P>
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A type specifier is returned, indicating the element type
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of the most specialized complex number representation capable of having
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parts of the specified argument <i>type</i>.
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The result is necessarily a supertype of the given <i>type</i>.
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Furthermore, if a type <i>A</i> is a subtype of type <i>B</i>, then
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<tt>(upgraded-complex-part-type <i>A</i>)</tt> is a subtype of
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<tt>(upgraded-complex-part-type <i>B</i>)</tt>.
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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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<P>
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<BR> <HR><A NAME=tex2html2173 HREF="node55.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2171 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2167 HREF="node53.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2175 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2176 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2174 HREF="node55.html"> Program Structure</A>
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<B>Up:</B> <A NAME=tex2html2172 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2168 HREF="node53.html"> Determining the Type </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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