85 lines
5.2 KiB
HTML
85 lines
5.2 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>6. Predicates</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Predicates">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html2368 HREF="node70.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2366 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2360 HREF="node68.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2370 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2371 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html2369 HREF="node70.html"> Logical Values</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html2367 HREF="clm.html">Common Lisp the Language</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html2361 HREF="node68.html"> Control of Time </A>
|
||
|
|
<HR> <P>
|
||
|
|
<H1><A NAME=SECTION001000000000000000000>6. Predicates</A></H1>
|
||
|
|
<P>
|
||
|
|
<A NAME=PREDS>A</A>
|
||
|
|
<i>predicate</i> is a function that tests for some condition involving
|
||
|
|
its arguments and returns <tt>nil</tt> if the condition is false, or some
|
||
|
|
non-<tt>nil</tt> value if the condition is true. One may think of a predicate as
|
||
|
|
producing a Boolean value, where <tt>nil</tt> stands for <i>false</i> and anything
|
||
|
|
else stands for <i>true</i>. Conditional control structures such as
|
||
|
|
<tt>cond</tt>,
|
||
|
|
<tt>if</tt>, <tt>when</tt>, and <tt>unless</tt> test such Boolean values.
|
||
|
|
We say that a predicate <i>is true</i> when it returns a non-<tt>nil</tt> value,
|
||
|
|
and <i>is false</i> when it returns <tt>nil</tt>; that is, it is true or false
|
||
|
|
according to whether the condition being tested is true or false.
|
||
|
|
<P>
|
||
|
|
By convention, the names of predicates usually end in the letter
|
||
|
|
<tt>p</tt> (which stands for ``predicate'').
|
||
|
|
Common Lisp uses a uniform convention in hyphenating names of predicates.
|
||
|
|
If the name of the predicate is formed by adding a <tt>p</tt> to
|
||
|
|
an existing name, such as the name of a data type,
|
||
|
|
a hyphen is placed before the final <tt>p</tt> if and only if there is
|
||
|
|
a hyphen in the existing name. For example, <tt>number</tt> begets <tt>numberp</tt>
|
||
|
|
but <tt>standard-char</tt> begets <tt>standard-char-p</tt>.
|
||
|
|
On the other hand, if the name of a predicate is formed by adding
|
||
|
|
a prefixing qualifier to the front of an existing predicate name,
|
||
|
|
the two names are joined with a hyphen and the presence or absence
|
||
|
|
of a hyphen before the final <tt>p</tt> is not changed. For example,
|
||
|
|
the predicate <tt>string-lessp</tt> has no hyphen before the <tt>p</tt>
|
||
|
|
because it is the string version of <tt>lessp</tt> (a MacLisp function
|
||
|
|
that has been renamed <tt><</tt> in Common Lisp). The name <tt>string-less-p</tt>
|
||
|
|
would incorrectly imply that it is a predicate that tests for a kind
|
||
|
|
of object called a <tt>string-less</tt>, and the name <tt>stringlessp</tt>
|
||
|
|
would connote a predicate that tests whether something has no strings
|
||
|
|
(is ``stringless'')!
|
||
|
|
<P>
|
||
|
|
The control structures that test Boolean values only test for
|
||
|
|
whether or not the value is <tt>nil</tt>, which is considered to be false. Any
|
||
|
|
other value is considered to be true. Often a predicate will return <tt>nil</tt> if
|
||
|
|
it ``fails'' and some <i>useful</i> value if it ``succeeds'';
|
||
|
|
such a function can be used not only as a test but
|
||
|
|
also for the useful value provided in case of success. An example
|
||
|
|
is <tt>member</tt>.
|
||
|
|
<P>
|
||
|
|
If no better non-<tt>nil</tt> value is available for the purpose of indicating
|
||
|
|
success, by convention the symbol <tt>t</tt> is used as the ``standard''
|
||
|
|
true value.
|
||
|
|
<P>
|
||
|
|
<HR>
|
||
|
|
<UL>
|
||
|
|
<LI> <A NAME=tex2html2372 HREF="node70.html#SECTION001010000000000000000"> Logical Values</A>
|
||
|
|
<LI> <A NAME=tex2html2373 HREF="node71.html#SECTION001020000000000000000"> Data Type Predicates</A>
|
||
|
|
<UL>
|
||
|
|
<LI> <A NAME=tex2html2374 HREF="node72.html#SECTION001021000000000000000"> General Type Predicates</A>
|
||
|
|
<LI> <A NAME=tex2html2375 HREF="node73.html#SECTION001022000000000000000"> Specific Data Type Predicates</A>
|
||
|
|
</UL>
|
||
|
|
<LI> <A NAME=tex2html2376 HREF="node74.html#SECTION001030000000000000000"> Equality Predicates</A>
|
||
|
|
<LI> <A NAME=tex2html2377 HREF="node75.html#SECTION001040000000000000000"> Logical Operators</A>
|
||
|
|
</UL>
|
||
|
|
<BR> <HR><A NAME=tex2html2368 HREF="node70.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2366 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2360 HREF="node68.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2370 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2371 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html2369 HREF="node70.html"> Logical Values</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html2367 HREF="clm.html">Common Lisp the Language</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html2361 HREF="node68.html"> Control of Time </A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|