HCoop / hcoop / debian / mlton.git / blame
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| 22 | <a href="./Home">MLton 20180207</a>\r | |
| 23 | </div>\r | |
| 24 | </div>\r | |
| 25 | <div id="header">\r | |
| 26 | <h1>PropertyList</h1>\r | |
| 27 | </div>\r | |
| 28 | <div id="content">\r | |
| 29 | <div id="preamble">\r | |
| 30 | <div class="sectionbody">\r | |
| 31 | <div class="paragraph"><p>A property list is a dictionary-like data structure into which\r | |
| 32 | properties (name-value pairs) can be inserted and from which\r | |
| 33 | properties can be looked up by name. The term comes from the Lisp\r | |
| 34 | language, where every symbol has a property list for storing\r | |
| 35 | information, and where the names are typically symbols and keys can be\r | |
| 36 | any type of value.</p></div>\r | |
| 37 | <div class="paragraph"><p>Here is an SML signature for property lists such that for any type of\r | |
| 38 | value a new property can be dynamically created to manipulate that\r | |
| 39 | type of value in a property list.</p></div>\r | |
| 40 | <div class="listingblock">\r | |
| 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>\r | |
| 42 | <span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
| 43 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
| 44 | \r | |
| 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">-></span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
| 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">-></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">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"></span>\r | |
| 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">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">option</span><span class="p">}</span><span class="w"></span>\r | |
| 48 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 49 | </pre></div></div></div>\r | |
| 50 | <div class="paragraph"><p>Here is a functor demonstrating the use of property lists. It first\r | |
| 51 | creates a property list, then two new properties (of different types),\r | |
| 52 | and adds a value to the list for each property.</p></div>\r | |
| 53 | <div class="listingblock">\r | |
| 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>\r | |
| 55 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
| 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>\r | |
| 57 | \r | |
| 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">-></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>\r | |
| 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">-></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>\r | |
| 60 | \r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 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">" "</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">"</span><span class="se">\n</span><span class="s">"</span><span class="p">])</span><span class="w"></span>\r | |
| 66 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 67 | </pre></div></div></div>\r | |
| 68 | <div class="paragraph"><p>Applied to an appropriate implementation <span class="monospaced">PROPERTY_LIST</span>, the <span class="monospaced">Test</span>\r | |
| 69 | functor will produce the following output.</p></div>\r | |
| 70 | <div class="listingblock">\r | |
| 71 | <div class="content monospaced">\r | |
| 72 | <pre>13 17.0</pre>\r | |
| 73 | </div></div>\r | |
| 74 | </div>\r | |
| 75 | </div>\r | |
| 76 | <div class="sect1">\r | |
| 77 | <h2 id="_implementation">Implementation</h2>\r | |
| 78 | <div class="sectionbody">\r | |
| 79 | <div class="paragraph"><p>Because property lists can hold values of any type, their\r | |
| 80 | implementation requires a <a href="UniversalType">UniversalType</a>. Given that, a property\r | |
| 81 | list is simply a list of elements of the universal type. Adding a\r | |
| 82 | property adds to the front of the list, and looking up a property\r | |
| 83 | scans the list.</p></div>\r | |
| 84 | <div class="listingblock">\r | |
| 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>\r | |
| 86 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
| 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>\r | |
| 88 | \r | |
| 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>\r | |
| 90 | \r | |
| 91 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">'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>\r | |
| 92 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
| 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>\r | |
| 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">'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>\r | |
| 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>\r | |
| 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>\r | |
| 97 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
| 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>\r | |
| 99 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 100 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 101 | </pre></div></div></div>\r | |
| 102 | <div class="paragraph"><p>If <span class="monospaced">U: UNIVERSAL_TYPE</span>, then we can test our code as follows.</p></div>\r | |
| 103 | <div class="listingblock">\r | |
| 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>\r | |
| 105 | </pre></div></div></div>\r | |
| 106 | <div class="paragraph"><p>Of course, a serious implementation of property lists would have to\r | |
| 107 | handle duplicate insertions of the same property, as well as the\r | |
| 108 | removal of elements in order to avoid space leaks.</p></div>\r | |
| 109 | </div>\r | |
| 110 | </div>\r | |
| 111 | <div class="sect1">\r | |
| 112 | <h2 id="_also_see">Also see</h2>\r | |
| 113 | <div class="sectionbody">\r | |
| 114 | <div class="ulist"><ul>\r | |
| 115 | <li>\r | |
| 116 | <p>\r | |
| 117 | MLton relies heavily on property lists for attaching information to\r | |
| 118 | syntax tree nodes in its intermediate languages. See\r | |
| 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\r | |
| 120 | <a href="https://github.com/MLton/mlton/blob/master/lib/mlton/basic/property-list.fun"><span class="monospaced">property-list.fun</span></a>.\r | |
| 121 | </p>\r | |
| 122 | </li>\r | |
| 123 | <li>\r | |
| 124 | <p>\r | |
| 125 | The <a href="MLRISCLibrary">MLRISCLibrary</a> <a href="References#LeungGeorge99"> uses property lists\r | |
| 126 | extensively</a>.\r | |
| 127 | </p>\r | |
| 128 | </li>\r | |
| 129 | </ul></div>\r | |
| 130 | </div>\r | |
| 131 | </div>\r | |
| 132 | </div>\r | |
| 133 | <div id="footnotes"><hr></div>\r | |
| 134 | <div id="footer">\r | |
| 135 | <div id="footer-text">\r | |
| 136 | </div>\r | |
| 137 | <div id="footer-badges">\r | |
| 138 | </div>\r | |
| 139 | </div>\r | |
| 140 | </body>\r | |
| 141 | </html>\r |