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<a href="./Home">MLton 20201002</a>
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<h1>PropertyList</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>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></div>
<div class="paragraph"><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></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="k">signature</span><span class="w"> </span><span class="n">PROPERTY_LIST</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w">   </span><span class="k">sig</span><span class="w"></span>
<span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>

<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">new</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">newProperty</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="p">{</span><span class="n">add</span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"></span>
<span class="w">                                </span><span class="n">peek</span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">option</span><span class="p">}</span><span class="w"></span>
<span class="w">   </span><span class="k">end</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><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></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="k">functor</span><span class="w"> </span><span class="n">Test</span><span class="w"> </span><span class="p">(</span><span class="n">P</span><span class="p">:</span><span class="w"> </span><span class="n">PROPERTY_LIST</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w">   </span><span class="k">struct</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">pl</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">P</span><span class="p">.</span><span class="n">new</span><span class="w"> </span><span class="p">()</span><span class="w"></span>

<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="p">{</span><span class="n">add</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">addInt</span><span class="p">:</span><span class="w"> </span><span class="n">P</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"> </span><span class="n">peek</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">peekInt</span><span class="p">}</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">P</span><span class="p">.</span><span class="n">newProperty</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="p">{</span><span class="n">add</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">addReal</span><span class="p">:</span><span class="w"> </span><span class="n">P</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"> </span><span class="n">peek</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">peekReal</span><span class="p">}</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">P</span><span class="p">.</span><span class="n">newProperty</span><span class="w"> </span><span class="p">()</span><span class="w"></span>

<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">addInt</span><span class="w"> </span><span class="p">(</span><span class="n">pl</span><span class="p">,</span><span class="w"> </span><span class="mi">13</span><span class="p">)</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">addReal</span><span class="w"> </span><span class="p">(</span><span class="n">pl</span><span class="p">,</span><span class="w"> </span><span class="mf">17.0</span><span class="p">)</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">s1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="p">(</span><span class="n">valOf</span><span class="w"> </span><span class="p">(</span><span class="n">peekInt</span><span class="w"> </span><span class="n">pl</span><span class="p">))</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">s2</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="p">(</span><span class="n">valOf</span><span class="w"> </span><span class="p">(</span><span class="n">peekReal</span><span class="w"> </span><span class="n">pl</span><span class="p">))</span><span class="w"></span>
<span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="n">concat</span><span class="w"> </span><span class="p">[</span><span class="n">s1</span><span class="p">,</span><span class="w"> </span><span class="s">&quot; &quot;</span><span class="p">,</span><span class="w"> </span><span class="n">s2</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">])</span><span class="w"></span>
<span class="w">   </span><span class="k">end</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>Applied to an appropriate implementation <span class="monospaced">PROPERTY_LIST</span>, the <span class="monospaced">Test</span>
functor will produce the following output.</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>13 17.0</pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_implementation">Implementation</h2>
<div class="sectionbody">
<div class="paragraph"><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></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="k">functor</span><span class="w"> </span><span class="n">PropertyList</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">:</span><span class="w"> </span><span class="n">UNIVERSAL_TYPE</span><span class="p">):</span><span class="w"> </span><span class="n">PROPERTY_LIST</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w">   </span><span class="k">struct</span><span class="w"></span>
<span class="w">      </span><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">U</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="n">ref</span><span class="w"></span>

<span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">new</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">ref</span><span class="w"> </span><span class="p">[])</span><span class="w"></span>

<span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">newProperty</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w">         </span><span class="k">let</span><span class="w"></span>
<span class="w">            </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">inject</span><span class="p">,</span><span class="w"> </span><span class="n">out</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">U</span><span class="p">.</span><span class="n">embed</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
<span class="w">            </span><span class="k">fun</span><span class="w"> </span><span class="n">add</span><span class="w"> </span><span class="p">(</span><span class="n">T</span><span class="w"> </span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">:</span><span class="w"> </span><span class="n">&#39;a</span><span class="p">):</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">inject</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="p">(</span><span class="n">!r</span><span class="p">)</span><span class="w"></span>
<span class="w">            </span><span class="k">fun</span><span class="w"> </span><span class="n">peek</span><span class="w"> </span><span class="p">(</span><span class="n">T</span><span class="w"> </span><span class="n">r</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w">               </span><span class="n">Option</span><span class="p">.</span><span class="n">map</span><span class="w"> </span><span class="p">(</span><span class="n">valOf</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">out</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">List</span><span class="p">.</span><span class="n">find</span><span class="w"> </span><span class="p">(</span><span class="n">isSome</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">out</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">!r</span><span class="p">))</span><span class="w"></span>
<span class="w">         </span><span class="k">in</span><span class="w"></span>
<span class="w">            </span><span class="p">{</span><span class="n">add</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">add</span><span class="p">,</span><span class="w"> </span><span class="n">peek</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">peek</span><span class="p">}</span><span class="w"></span>
<span class="w">         </span><span class="k">end</span><span class="w"></span>
<span class="w">   </span><span class="k">end</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>If <span class="monospaced">U: UNIVERSAL_TYPE</span>, then we can test our code as follows.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="k">structure</span><span class="w"> </span><span class="n">Z</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Test</span><span class="w"> </span><span class="p">(</span><span class="n">PropertyList</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">))</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><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></div>
</div>
</div>
<div class="sect1">
<h2 id="_also_see">Also see</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
MLton relies heavily on property lists for attaching information to
syntax tree nodes in its intermediate languages.  See
<a href="https://github.com/MLton/mlton/blob/master/lib/mlton/basic/property-list.sig"><span class="monospaced">property-list.sig</span></a> and
<a href="https://github.com/MLton/mlton/blob/master/lib/mlton/basic/property-list.fun"><span class="monospaced">property-list.fun</span></a>.
</p>
</li>
<li>
<p>
The <a href="MLRISCLibrary">MLRISCLibrary</a> <a href="References#LeungGeorge99"> uses property lists
extensively</a>.
</p>
</li>
</ul></div>
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