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PropertyList
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A property list is a dictionary-like data structure into which properties (name-value pairs) can be inserted and from which properties can be looked up by name. The term comes from the Lisp language, where every symbol has a property list for storing information, and where the names are typically symbols and keys can be any type of value. <p>
Here is an SML signature for property lists such that for any type of value a new property can be dynamically created to manipulate that type of value in a property list.
</p>
<pre class=code>
<B><FONT COLOR="#0000FF">signature</FONT></B> PROPERTY_LIST =
<B><FONT COLOR="#0000FF">sig</FONT></B>
<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> new: unit -> t
<B><FONT COLOR="#A020F0">val</FONT></B> newProperty: unit -> {add: t * 'a -> unit,
peek: t -> 'a option}
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
</p>
<p>
Here is a functor demonstrating the use of property lists. It first creates a property list, then two new properties (of different types), and adds a value to the list for each property.
</p>
<pre class=code>
<B><FONT COLOR="#0000FF">functor</FONT></B> Test (P: PROPERTY_LIST) =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> pl = P.new ()
<B><FONT COLOR="#A020F0">val</FONT></B> {add = addInt: P.t * int -> unit, peek = peekInt} = P.newProperty ()
<B><FONT COLOR="#A020F0">val</FONT></B> {add = addReal: P.t * real -> unit, peek = peekReal} = P.newProperty ()
<B><FONT COLOR="#A020F0">val</FONT></B> () = addInt (pl, <B><FONT COLOR="#5F9EA0">13</FONT></B>)
<B><FONT COLOR="#A020F0">val</FONT></B> () = addReal (pl, <B><FONT COLOR="#5F9EA0">17.0</FONT></B>)
<B><FONT COLOR="#A020F0">val</FONT></B> s1 = Int.toString (valOf (peekInt pl))
<B><FONT COLOR="#A020F0">val</FONT></B> s2 = Real.toString (valOf (peekReal pl))
<B><FONT COLOR="#A020F0">val</FONT></B> () = print (concat [s1, <B><FONT COLOR="#BC8F8F">" "</FONT></B>, s2, <B><FONT COLOR="#BC8F8F">"\n"</FONT></B>])
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
</p>
<p>
Applied to an appropriate implementation <tt>PROPERTY_LIST</tt>, the <tt>Test</tt> functor will produce the following output.
<pre>13 17.0
</pre>
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<p>
Because property lists can hold values of any type, their implementation requires a <a href="UniversalType">UniversalType</a>. Given that, a property list is simply a list of elements of the universal type. Adding a property adds to the front of the list, and looking up a property scans the list.
</p>
<pre class=code>
<B><FONT COLOR="#0000FF">functor</FONT></B> PropertyList (U: UNIVERSAL_TYPE): PROPERTY_LIST =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> U.t list ref
</FONT></B><B><FONT COLOR="#A020F0">fun</FONT></B> new () = T (ref [])
<B><FONT COLOR="#A020F0">fun</FONT></B> <B><FONT COLOR="#228B22">'a</FONT></B> newProperty () =
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> (inject, out) = U.embed ()
<B><FONT COLOR="#A020F0">fun</FONT></B> add (T r, a: 'a): unit = r := inject a :: (!r)
<B><FONT COLOR="#A020F0">fun</FONT></B> peek (T r) =
Option.map (valOf o out) (List.find (isSome o out) (!r))
<B><FONT COLOR="#A020F0">in</FONT></B>
{add = add, peek = peek}
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
</p>
<p>
If <tt>U: UNIVERSAL_TYPE</tt>, then we can test our code as follows.
<pre class=code>
<B><FONT COLOR="#0000FF">structure</FONT></B> Z = Test (PropertyList (U))
</PRE>
</p>
<p>
Of course, a serious implementation of property lists would have to handle duplicate insertions of the same property, as well as the removal of elements in order to avoid space leaks.
</p>
<h2 id="head-a4bc8bf5caf54b18cea9f58e83dd4acb488deb17">Also see</h2>
<p>
MLton relies heavily on property lists for attaching information to syntax tree nodes in its intermediate languages. See <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/lib/mlton/basic/property-list.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">property-list.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/lib/mlton/basic/property-list.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">property-list.fun</a>.
</p>
<p>
The <a href="MLRISCLibrary">MLRISCLibrary</a> <a href = "References#LeungGeorge98">uses property lists extensively</a>.
</p>
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<hr>
Last edited on 2010-03-02 15:17:55 by <span title="fenrir.cs.rit.edu"><a href="MatthewFluet">MatthewFluet</a></span>.
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