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4.7 KiB
HTML
86 lines
4.7 KiB
HTML
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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>5.1.3. Special Forms</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Special Forms">
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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=tex2html2251 HREF="node60.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2249 HREF="node56.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2243 HREF="node58.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2253 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2254 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2252 HREF="node60.html"> Macros</A>
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<B>Up:</B> <A NAME=tex2html2250 HREF="node56.html"> Forms</A>
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<B> Previous:</B> <A NAME=tex2html2244 HREF="node58.html"> Variables</A>
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<HR> <P>
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<H2><A NAME=SECTION00913000000000000000>5.1.3. Special Forms</A></H2>
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<P>
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If a list is to be evaluated as a form, the first step is to examine
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the first element of the list. If the first element is one of
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the symbols appearing in table <A HREF="node59.html#SPECIALFORMTABLE">5-1</A>,
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then the list is called a <i>special form</i>. (This use of the word
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``special'' is unrelated to its use in the phrase ``special variable.'')
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<P>
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Special forms are generally environment and control constructs.
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Every special form has its own idiosyncratic syntax. An example
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is the <tt>if</tt> special form:
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<tt>(if p (+ x 4) 5)</tt> in Common Lisp means what
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``<b>if</b> <i>p</i> <b>then</b> <i>x</i>+4 <b>else</b> 5'' means in Algol.
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<P>
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The evaluation of a special form normally produces a value or values,
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but the evaluation may instead call for a non-local exit; see <tt>return-from</tt>,
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<tt>go</tt>, and <tt>throw</tt>.
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<P>
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The set of special forms is fixed in Common Lisp; no way is provided
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for the user to define more. The user can create new syntactic
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constructs, however, by defining macros.
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<P>
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The set of special forms in Common Lisp is purposely kept very small
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because any program-analyzing program must have special knowledge
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about every type of special form. Such a program needs no special
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knowledge about macros because it is simple to expand the macro
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and operate on the resulting expansion. (This is not to say that
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many such programs, particularly compilers, will not have such
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special knowledge. A compiler may be able
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to produce much better code if it recognizes such constructs
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as <tt>typecase</tt> and <tt>multiple-value-bind</tt> and gives them customized
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treatment.)
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<P>
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<A NAME=SPECIALFORMTABLE> </A><IMG ALIGN=BOTTOM ALT="" SRC="_24769_table3064.gif">
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<P>
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An implementation is free to implement as a macro any construct described
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herein as a special form. Conversely, an implementation is free
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to implement as a special form any construct described herein as a macro
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if an equivalent macro definition is also provided.
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The practical consequence is that the predicates <tt>macro-function</tt> and
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<tt>special-form-p</tt> may both be true of the same symbol.
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It is recommended that a program-analyzing program process
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a form that is a list whose <i>car</i> is a symbol as follows:
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<OL><LI>
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If the program has particular knowledge about the symbol,
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process the form using special-purpose code.
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All of the symbols listed in table <A HREF="node59.html#SPECIALFORMTABLE">5-1</A>
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should fall into this category.
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<P>
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<LI>
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Otherwise, if <tt>macro-function</tt> is true of the symbol, apply either
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<tt>macroexpand</tt> or <tt>macroexpand-1</tt>, as appropriate,
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to the entire form and then start over.
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<P>
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<LI>
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Otherwise, assume it is a function call.
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</OL>
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<P>
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<BR> <HR><A NAME=tex2html2251 HREF="node60.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2249 HREF="node56.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2243 HREF="node58.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2253 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2254 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2252 HREF="node60.html"> Macros</A>
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<B>Up:</B> <A NAME=tex2html2250 HREF="node56.html"> Forms</A>
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<B> Previous:</B> <A NAME=tex2html2244 HREF="node58.html"> Variables</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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