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293 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>10.1. The Property List</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" The Property List">
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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=tex2html2868 HREF="node109.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2866 HREF="node107.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2860 HREF="node107.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2870 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2871 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2869 HREF="node109.html"> The Print Name</A>
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<B>Up:</B> <A NAME=tex2html2867 HREF="node107.html"> Symbols</A>
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<B> Previous:</B> <A NAME=tex2html2861 HREF="node107.html"> Symbols</A>
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<HR> <P>
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<H1><A NAME=SECTION001410000000000000000>10.1. The Property List</A></H1>
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<P>
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Since its inception, Lisp has associated with each symbol
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a kind of tabular data structure
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called a <i>property list</i> (<i>plist</i> for short). A property list contains
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zero or more entries; each entry associates with a key
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(called the <i>indicator</i>), which is typically
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a symbol, an arbitrary Lisp object (called the <i>value</i> or,
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sometimes, the <i>property</i>).
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There are no duplications among the indicators; a property list may only
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have one property at a time with a given name. In this way, given
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a symbol and an indicator (another symbol), an associated value can be
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retrieved.
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<P>
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A property
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list is very similar in purpose to an association list. The difference
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is that a property list is an object with a unique identity; the
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operations for adding and removing property-list entries are destructive
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operations that alter the property list rather than making a new one.
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Association lists, on the other hand, are normally augmented
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non-destructively (without side effects) by adding new entries to the
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front (see <tt>acons</tt> and <tt>pairlis</tt>).
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<P>
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A property list is implemented as a memory cell
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containing a list with an even number (possibly zero) of elements.
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(Usually this memory cell is the property-list cell of a symbol,
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but any memory cell acceptable to <tt>setf</tt> can be used
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if <tt>getf</tt> and <tt>remf</tt> are used.)
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Each pair of elements in the list constitutes an entry;
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the first item is the indicator, and the second is the
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value. Because property-list functions are given the symbol
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and not the list itself, modifications to the property list
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can be recorded by storing back into the property-list cell of the symbol.
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<P>
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When a symbol is created, its property list is initially empty.
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Properties are created by using <tt>get</tt> within a <tt>setf</tt> form.
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<P>
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Common Lisp does not use a symbol's property list as extensively as earlier
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Lisp implementations did. Less-used data, such as compiler,
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debugging, and documentation information, is kept on property lists
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in Common Lisp.
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<P>
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<hr>
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<b>Compatibility note:</b> In older Lisp implementations, the
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print name, value, and function definition of a symbol were kept on its
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property list. The value cell was introduced into MacLisp and Interlisp
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to speed up access to variables; similarly for the
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print-name cell and function cell (MacLisp does not use a function cell).
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Recent Lisp implementations such as Spice Lisp,
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Lisp Machine Lisp, and NIL have
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introduced all of these cells plus the
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package cell.
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None of the MacLisp system property names
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(<tt>expr</tt>, <tt>fexpr</tt>, <tt>macro</tt>, <tt>array</tt>,
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<tt>subr</tt>, <tt>lsubr</tt>, <tt>fsubr</tt>, and in former times <tt>value</tt> and
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<tt>pname</tt>) exist in Common Lisp.
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<P>
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In Common Lisp, the notion of ``disembodied property list''
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introduced in MacLisp is eliminated. It tended to be used for
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rather kludgy things, and in Lisp Machine Lisp is often associated with
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the use of locatives (to make it ``off by one'' for searching
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alternating keyword lists). In Common Lisp special <tt>setf</tt>-like
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property-list functions are introduced: <tt>getf</tt>
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and <tt>remf</tt>.
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<hr>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>get <i>symbol</i> <i>indicator</i> &optional <i>default</i></tt><P><tt>get</tt> searches the property list of
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<i>symbol</i> for an indicator <tt>eq</tt> to <i>indicator</i>.
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The first argument must be a symbol.
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If one is found, then the corresponding value is returned;
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otherwise <i>default</i> is returned.
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<P>
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If <i>default</i> is not specified,
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then <tt>nil</tt> is used for <i>default</i>.
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<P>
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Note that there is no way to distinguish an absent property from
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one whose value is <i>default</i>.
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<P><pre>
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(get x y) == (getf (symbol-plist x) y)
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</pre><P>
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Suppose that the property list of <tt>foo</tt> is <tt>(bar t baz 3 hunoz "Huh?")</tt>.
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Then, for example:
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<P><pre>
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(get 'foo 'baz) => 3
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(get 'foo 'hunoz) => "Huh?"
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(get 'foo 'zoo) => <tt>nil</tt>
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</pre><P>
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<P>
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<hr>
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<b>Compatibility note:</b> In MacLisp, the first argument to <tt>get</tt> could
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be a list, in which case the <i>cdr</i> of the list was treated
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as a so-called ``disembodied property list.''
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The first argument to <tt>get</tt>
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could also be any other object, in which case <tt>get</tt> would
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always return <tt>nil</tt>. In Common Lisp, it is an error to give anything
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but a symbol as the first argument to <tt>get</tt>.
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<P>
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What Common Lisp calls <tt>get</tt>, Interlisp calls <tt>getprop</tt>.
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<P>
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What MacLisp and Interlisp call <tt>putprop</tt> is accomplished
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in Common Lisp by using <tt>get</tt> with <tt>setf</tt>.
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<hr>
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<P>
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<tt>setf</tt> may be used with <tt>get</tt> to create a new property-value
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pair, possibly replacing an old pair with the same property name.
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For example:
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<P><pre>
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(get 'clyde 'species) => <tt>nil</tt>
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(setf (get 'clyde 'species) 'elephant) => elephant
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and now (get 'clyde 'species) => elephant
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</pre><P>
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The <i>default</i> argument may be
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specified to <tt>get</tt> in this context; it is ignored by <tt>setf</tt> but
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may be useful in such macros as <tt>push</tt> that are related to <tt>setf</tt>:
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<P><pre>
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(push item (get sym 'token-stack '(initial-item)))
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</pre><P>
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means approximately the same as
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<P><pre>
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(setf (get sym 'token-stack '(initial-item))
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(cons item (get sym 'token-stack '(initial-item))))
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</pre><P>
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which in turn would be treated as simply
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<P><pre>
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(setf (get sym 'token-stack)
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(cons item (get sym 'token-stack '(initial-item))))
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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 1989 (REMF-DESTRUCTION-UNSPECIFIED) <A NAME=9866> </A>
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to clarify the permissible side effects of certain operations;
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<tt>(setf (get <i>symbol</i> <i>indicator</i>) <i>newvalue</i>)</tt>
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is required to behave exactly the same as
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<tt>(setf (getf (symbol-plist <i>symbol</i>) <i>indicator</i>) <i>newvalue</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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<BR><b>[Function]</b><BR>
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<tt>remprop <i>symbol</i> <i>indicator</i></tt><P>This removes from <i>symbol</i> the property with an indicator <tt>eq</tt>
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to <i>indicator</i>. The property indicator and the corresponding
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value are removed by destructively splicing the property
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list. It returns <tt>nil</tt> if no such property was found,
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or non-<tt>nil</tt> if a property was found.
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<P><pre>
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(remprop x y) == (remf (symbol-plist x) y)
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</pre><P>
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For example, if the property list of <tt>foo</tt> is initially
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<P><pre>
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(color blue height 6.3 near-to bar)
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</pre><P>
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then the call
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<P><pre>
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(remprop 'foo 'height)
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</pre><P>
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returns a non-<tt>nil</tt> value after altering <tt>foo</tt>'s property list to be
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<P><pre>
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(color blue near-to bar)
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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 1989 (REMF-DESTRUCTION-UNSPECIFIED) <A NAME=9893> </A>
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to clarify the permissible side effects of certain operations;
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<tt>(remprop <i>symbol</i> <i>indicator</i>)</tt>
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is required to behave exactly the same as
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<tt>(remf (symbol-plist <i>symbol</i>) <i>indicator</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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<BR><b>[Function]</b><BR>
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<tt>symbol-plist <i>symbol</i></tt><P>This returns the list that contains the property pairs of <i>symbol</i>;
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the contents of the property-list cell are extracted and returned.
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<P>
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Note that using <tt>get</tt> on the result of <tt>symbol-plist</tt> does <i>not</i> work.
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One must give the symbol itself to <tt>get</tt> or else
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use the function <tt>getf</tt>.
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<P>
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<tt>setf</tt> may be used with <tt>symbol-plist</tt> to destructively replace
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the entire property list of a symbol. This is a relatively dangerous
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operation, as it may destroy important information that
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the implementation may happen to store in property lists.
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Also, care must be taken that the new
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property list is in fact a list of even length.
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<p>
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<hr>
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<b>Compatibility note:</b> In MacLisp, this function is called <tt>plist</tt>;
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in Interlisp, it is called <tt>getproplist</tt>.
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<hr>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>getf <i>place</i> <i>indicator</i> &optional <i>default</i></tt><P><tt>getf</tt> searches the property list stored in <i>place</i>
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for an indicator <tt>eq</tt> to <i>indicator</i>.
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If one is found, then the corresponding value is returned;
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otherwise <i>default</i> is returned. If <i>default</i> is not specified,
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then <tt>nil</tt> is used for <i>default</i>.
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Note that there is no way to distinguish an absent property from
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one whose value is <i>default</i>.
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Often <i>place</i> is computed from
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a generalized variable acceptable to <tt>setf</tt>.
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<P>
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<tt>setf</tt> may be used with <tt>getf</tt>, in which case the <i>place</i> must
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indeed be acceptable as a <i>place</i> to <tt>setf</tt>. The effect is to
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add a new property-value pair, or update an existing pair,
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in the property list kept in the <i>place</i>.
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The <i>default</i> argument may be
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specified to <tt>getf</tt> in this context; it is ignored by <tt>setf</tt> but
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may be useful in such macros as <tt>push</tt> that are related to <tt>setf</tt>.
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See the description of <tt>get</tt> for an example of this.
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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 1989 (REMF-DESTRUCTION-UNSPECIFIED) <A NAME=9939> </A>
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to clarify the permissible side effects of certain operations;
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<tt>setf</tt> used with <tt>getf</tt> is permitted to perform a <tt>setf</tt>
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on the <i>place</i> or on any part, <i>car</i> or <i>cdr</i>, of the
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top-level list structure held by that <i>place</i>.
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<P>
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X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) <A NAME=9949> </A>
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to clarify order of evaluation (see section <A HREF="node80.html#SETFSECTION">7.2</A>).
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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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<hr>
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<b>Compatibility note:</b> The Interlisp function <tt>listget</tt> is similar to <tt>getf</tt>.
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The Interlisp function <tt>listput</tt> is similar to using <tt>getf</tt>
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with <tt>setf</tt>.
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<hr>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>remf <i>place</i> <i>indicator</i></tt><P>This removes from the property list stored in <i>place</i>
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the property with an indicator <tt>eq</tt>
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to <i>indicator</i>. The property indicator and the corresponding
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value are removed by destructively splicing the property
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list. <tt>remf</tt> returns <tt>nil</tt> if no such property was found,
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or some non-<tt>nil</tt> value if a property was found.
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The form <i>place</i> may be any generalized variable acceptable to <tt>setf</tt>.
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See <tt>remprop</tt>.
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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 1989 (REMF-DESTRUCTION-UNSPECIFIED) <A NAME=9971> </A>
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to clarify the permissible side effects of certain operations;
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<tt>remf</tt> is permitted to perform a <tt>setf</tt>
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on the <i>place</i> or on any part, <i>car</i> or <i>cdr</i>, of the
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top-level list structure held by that <i>place</i>.
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<P>
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X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) <A NAME=9980> </A>
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to clarify order of evaluation (see section <A HREF="node80.html#SETFSECTION">7.2</A>).
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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><b>[Function]</b><BR>
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<tt>get-properties <i>place</i> <i>indicator-list</i></tt><P><tt>get-properties</tt> is like <tt>getf</tt>, except that the second
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argument is a list of indicators. <tt>get-properties</tt> searches the
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property list stored in <i>place</i> for any of the indicators in
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<i>indicator-list</i> until it finds the first property
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in the property list whose indicator is one of
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the elements of <i>indicator-list</i>. Normally <i>place</i> is computed from
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a generalized variable acceptable to <tt>setf</tt>.
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<P>
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<tt>get-properties</tt> returns three values.
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If any property was found, then
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the first two values are the indicator
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and value for the first property whose indicator was in <i>indicator-list</i>,
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and the third is that tail of the property list whose <i>car</i>
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was the indicator (and whose <i>cadr</i> is therefore the value).
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If no property was found, all three values are <tt>nil</tt>.
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Thus the third value serves as a flag indicating success or failure
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and also allows the search to be restarted, if desired, after the property
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was found.
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