doc/guide/localhost/PropertyList

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  26 <h1>PropertyList</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>A property list is a dictionary-like data structure into which
  32 properties (name-value pairs) can be inserted and from which
  33 properties can be looked up by name.  The term comes from the Lisp
  34 language, where every symbol has a property list for storing
  35 information, and where the names are typically symbols and keys can be
  36 any type of value.</p></div>
  37 <div class="paragraph"><p>Here is an SML signature for property lists such that for any type of
  38 value a new property can be dynamically created to manipulate that
  39 type of value in a property list.</p></div>
  40 <div class="listingblock">
  41 <div class="content"><div class="highlight"><pre><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>
  42 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
  43 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
  44
  45 <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>
  46 <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>
  47 <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>
  48 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  49 </pre></div></div></div>
  50 <div class="paragraph"><p>Here is a functor demonstrating the use of property lists.  It first
  51 creates a property list, then two new properties (of different types),
  52 and adds a value to the list for each property.</p></div>
  53 <div class="listingblock">
  54 <div class="content"><div class="highlight"><pre><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>
  55 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  56 <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>
  57
  58 <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>
  59 <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>
  60
  61 <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>
  62 <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>
  63 <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>
  64 <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>
  65 <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>
  66 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  67 </pre></div></div></div>
  68 <div class="paragraph"><p>Applied to an appropriate implementation <span class="monospaced">PROPERTY_LIST</span>, the <span class="monospaced">Test</span>
  69 functor will produce the following output.</p></div>
  70 <div class="listingblock">
  71 <div class="content monospaced">
  72 <pre>13 17.0</pre>
  73 </div></div>
  74 </div>
  75 </div>
  76 <div class="sect1">
  77 <h2 id="_implementation">Implementation</h2>
  78 <div class="sectionbody">
  79 <div class="paragraph"><p>Because property lists can hold values of any type, their
  80 implementation requires a <a href="UniversalType">UniversalType</a>.  Given that, a property
  81 list is simply a list of elements of the universal type.  Adding a
  82 property adds to the front of the list, and looking up a property
  83 scans the list.</p></div>
  84 <div class="listingblock">
  85 <div class="content"><div class="highlight"><pre><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>
  86 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  87 <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>
  88
  89 <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>
  90
  91 <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>
  92 <span class="w">         </span><span class="k">let</span><span class="w"></span>
  93 <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>
  94 <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>
  95 <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>
  96 <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>
  97 <span class="w">         </span><span class="k">in</span><span class="w"></span>
  98 <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>
  99 <span class="w">         </span><span class="k">end</span><span class="w"></span>
 100 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 101 </pre></div></div></div>
 102 <div class="paragraph"><p>If <span class="monospaced">U: UNIVERSAL_TYPE</span>, then we can test our code as follows.</p></div>
 103 <div class="listingblock">
 104 <div class="content"><div class="highlight"><pre><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>
 105 </pre></div></div></div>
 106 <div class="paragraph"><p>Of course, a serious implementation of property lists would have to
 107 handle duplicate insertions of the same property, as well as the
 108 removal of elements in order to avoid space leaks.</p></div>
 109 </div>
 110 </div>
 111 <div class="sect1">
 112 <h2 id="_also_see">Also see</h2>
 113 <div class="sectionbody">
 114 <div class="ulist"><ul>
 115 <li>
 116 <p>
 117 MLton relies heavily on property lists for attaching information to
 118 syntax tree nodes in its intermediate languages.  See
 119 <a href="https://github.com/MLton/mlton/blob/master/lib/mlton/basic/property-list.sig"><span class="monospaced">property-list.sig</span></a> and
 120 <a href="https://github.com/MLton/mlton/blob/master/lib/mlton/basic/property-list.fun"><span class="monospaced">property-list.fun</span></a>.
 121 </p>
 122 </li>
 123 <li>
 124 <p>
 125 The <a href="MLRISCLibrary">MLRISCLibrary</a> <a href="References#LeungGeorge99"> uses property lists
 126 extensively</a>.
 127 </p>
 128 </li>
 129 </ul></div>
 130 </div>
 131 </div>
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