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107 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>4.7. Defining New Type Specifiers</TITLE>
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<BODY>
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<meta name="description" value=" Defining New Type Specifiers">
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<meta name="keywords" value="clm">
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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=tex2html2139 HREF="node52.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2137 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2131 HREF="node50.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2141 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2142 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2140 HREF="node52.html"> Type Conversion Function</A>
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<B>Up:</B> <A NAME=tex2html2138 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2132 HREF="node50.html"> Type Specifiers That </A>
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<HR> <P>
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<H1><A NAME=SECTION00870000000000000000>4.7. Defining New Type Specifiers</A></H1>
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<P>
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New type specifiers can come into existence in two ways.
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First, defining a new structure type with <tt>defstruct</tt> automatically
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causes the name of the structure to be a new type specifier symbol.
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Second, the <tt>deftype</tt> special form can be used to define new type-specifier
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abbreviations.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>deftype</tt> <tt><I>name</I></tt> <tt><I>lambda-list</I></tt> <b>[[</b><tt>{<I>declaration</I>}*</tt> <tt>|</tt> <tt><I>doc-string</I></tt><b>]]</b> <tt>{<I>form</I>}*</tt><P>
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This is very similar to a <tt>defmacro</tt> form: <i>name</i> is the
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symbol that identifies the type specifier being defined, <i>lambda-list</i> is
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a lambda-list (and may contain <tt>&optional</tt> and <tt>&rest</tt>
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markers), and
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the <i>forms</i> constitute the body of the expander function. If we view a
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type specifier list as a list containing the type specifier name and some argument forms,
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the argument forms (unevaluated) are bound to the corresponding
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parameters in <i>lambda-list</i>. Then the body forms are evaluated
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as an implicit <tt>progn</tt>, and the value of the last form
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is interpreted as a new type specifier for which the original specifier
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was an abbreviation. The <i>name</i> is returned as the value of the
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<tt>deftype</tt> form.
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<P>
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<tt>deftype</tt> differs from <tt>defmacro</tt> in that if no <i>initform</i>
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is specified for an <tt>&optional</tt> parameter, the default value
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is <tt>*</tt>, not <tt>nil</tt>.
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<P>
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If the optional documentation string <i>doc-string</i> is present,
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then it is attached to the <i>name</i>
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as a documentation string of type <tt>type</tt>; see <tt>documentation</tt>.
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<P>
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Here are some examples of the use of <tt>deftype</tt>:
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<P><pre>
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(deftype mod (n) `(integer 0 (,n))) <BR><BR>(deftype list () '(or null cons))
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</pre><P>
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<P>
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<P>
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<P><pre>
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(deftype square-matrix (<tt>&optional</tt> type size)
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"SQUARE-MATRIX includes all square two-dimensional arrays."
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`(array ,type (,size ,size)))
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(square-matrix short-float 7) means (array short-float (7 7))
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(square-matrix bit) means (array bit (* *))
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</pre><P>
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If the type name defined by <tt>deftype</tt> is used simply as a type
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specifier symbol, it is interpreted as a type specifier list with
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no argument forms. Thus, in the example above, <tt>square-matrix</tt>
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would mean <tt>(array * (* *))</tt>, the set of two-dimensional arrays.
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This would unfortunately fail to convey the constraint that the two
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dimensions be the same; <tt>(square-matrix bit)</tt> has the same problem.
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A better definition is
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<P><pre>
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(defun equidimensional (a)
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(or (< (array-rank a) 2)
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(apply #'= (array-dimensions a))))
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(deftype square-matrix (<tt>&optional</tt> type size)
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`(and (array ,type (,size ,size))
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(satisfies equidimensional)))
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</pre><P>
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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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X3J13 voted in March 1988 (FLET-IMPLICIT-BLOCK) <A NAME=2641> </A>
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to specify that the body of the expander function defined
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by <tt>deftype</tt> is implicitly enclosed in a <tt>block</tt> construct
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whose name is the same as the <i>name</i> of the defined type.
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Therefore <tt>return-from</tt> may be used to exit from the function.
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<P>
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X3J13 voted in March 1989 (DEFINING-MACROS-NON-TOP-LEVEL) <A NAME=2648> </A>
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to clarify that, while defining forms normally appear at top level,
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it is meaningful to place them in non-top-level contexts;
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<tt>deftype</tt> must define the expander function
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within the enclosing lexical environment, not within the global
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environment.
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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> <HR><A NAME=tex2html2139 HREF="node52.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2137 HREF="node44.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2131 HREF="node50.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2141 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2142 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2140 HREF="node52.html"> Type Conversion Function</A>
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<B>Up:</B> <A NAME=tex2html2138 HREF="node44.html"> Type Specifiers</A>
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<B> Previous:</B> <A NAME=tex2html2132 HREF="node50.html"> Type Specifiers That </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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