doc/guide/localhost/AST

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>AST</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="AST">AST</a> is the <a href="IntermediateLanguage">IntermediateLanguage</a> produced by the <a href="FrontEnd">FrontEnd</a>
  32 and translated by <a href="Elaborate">Elaborate</a> to <a href="CoreML">CoreML</a>.</p></div>
  33 </div>
  34 </div>
  35 <div class="sect1">
  36 <h2 id="_description">Description</h2>
  37 <div class="sectionbody">
  38 <div class="paragraph"><p>The abstract syntax tree produced by the <a href="FrontEnd">FrontEnd</a>.</p></div>
  39 </div>
  40 </div>
  41 <div class="sect1">
  42 <h2 id="_implementation">Implementation</h2>
  43 <div class="sectionbody">
  44 <div class="ulist"><ul>
  45 <li>
  46 <p>
  47 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-programs.sig"><span class="monospaced">ast-programs.sig</span></a>
  48 </p>
  49 </li>
  50 <li>
  51 <p>
  52 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-programs.fun"><span class="monospaced">ast-programs.fun</span></a>
  53 </p>
  54 </li>
  55 <li>
  56 <p>
  57 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-modules.sig"><span class="monospaced">ast-modules.sig</span></a>
  58 </p>
  59 </li>
  60 <li>
  61 <p>
  62 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-modules.fun"><span class="monospaced">ast-modules.fun</span></a>
  63 </p>
  64 </li>
  65 <li>
  66 <p>
  67 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-core.sig"><span class="monospaced">ast-core.sig</span></a>
  68 </p>
  69 </li>
  70 <li>
  71 <p>
  72 <a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-core.fun"><span class="monospaced">ast-core.fun</span></a>
  73 </p>
  74 </li>
  75 <li>
  76 <p>
  77 <a href="https://github.com/MLton/mlton/tree/master/mlton/ast"><span class="monospaced">ast</span></a>
  78 </p>
  79 </li>
  80 </ul></div>
  81 </div>
  82 </div>
  83 <div class="sect1">
  84 <h2 id="_type_checking">Type Checking</h2>
  85 <div class="sectionbody">
  86 <div class="paragraph"><p>The <a href="AST">AST</a> <a href="IntermediateLanguage">IntermediateLanguage</a> has no independent type
  87 checker. Type inference is performed on an AST program as part of
  88 <a href="Elaborate">Elaborate</a>.</p></div>
  89 </div>
  90 </div>
  91 <div class="sect1">
  92 <h2 id="_details_and_notes">Details and Notes</h2>
  93 <div class="sectionbody">
  94 <div class="sect2">
  95 <h3 id="_source_locations">Source locations</h3>
  96 <div class="paragraph"><p>MLton makes use of a relatively clean method for annotating the
  97 abstract syntax tree with source location information.  Every source
  98 program phrase is "wrapped" with the <span class="monospaced">WRAPPED</span> interface:</p></div>
  99 <div class="listingblock">
 100 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">WRAPPED</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 101 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 102 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"></span>
 103 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">obj</span><span class="w"></span>
 104
 105 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">dest</span><span class="p">:</span><span class="w"> </span><span class="n">obj</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">Region</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 106 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">makeRegion&#39;</span><span class="p">:</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">SourcePos</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">SourcePos</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">obj</span><span class="w"></span>
 107 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">makeRegion</span><span class="p">:</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">Region</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">obj</span><span class="w"></span>
 108 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">node</span><span class="p">:</span><span class="w"> </span><span class="n">obj</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"></span>
 109 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">region</span><span class="p">:</span><span class="w"> </span><span class="n">obj</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">Region</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 110 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 111 </pre></div></div></div>
 112 <div class="paragraph"><p>The key idea is that <span class="monospaced">node'</span> is the type of an unannotated syntax
 113 phrase and <span class="monospaced">obj</span> is the type of its annotated counterpart. In the
 114 implementation, every <span class="monospaced">node'</span> is annotated with a <span class="monospaced">Region.t</span>
 115 (<a href="https://github.com/MLton/mlton/blob/master/mlton/control/region.sig"><span class="monospaced">region.sig</span></a>,
 116 <a href="https://github.com/MLton/mlton/blob/master/mlton/control/region.sml"><span class="monospaced">region.sml</span></a>), which describes the
 117 syntax phrase&#8217;s left source position and right source position, where
 118 <span class="monospaced">SourcePos.t</span> (<a href="https://github.com/MLton/mlton/blob/master/mlton/control/source-pos.sig"><span class="monospaced">source-pos.sig</span></a>,
 119 <a href="https://github.com/MLton/mlton/blob/master/mlton/control/source-pos.sml"><span class="monospaced">source-pos.sml</span></a>) denotes a
 120 particular file, line, and column.  A typical use of the <span class="monospaced">WRAPPED</span>
 121 interface is illustrated by the following code:</p></div>
 122 <div class="listingblock">
 123 <div class="content"><div class="highlight"><pre><span class="w">            </span><span class="k">datatype</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 124 <span class="w">               </span><span class="n">App</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">Longcon</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">t</span><span class="w"></span>
 125 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Const</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">Const</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 126 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Constraint</span><span class="w"> </span><span class="k">of</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">Type</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 127 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">FlatApp</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 128 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Layered</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">constraint</span><span class="p">:</span><span class="w"> </span><span class="n">Type</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">option</span><span class="p">,</span><span class="w"></span>
 129 <span class="w">                           </span><span class="n">fixop</span><span class="p">:</span><span class="w"> </span><span class="n">Fixop</span><span class="p">.</span><span class="n">t</span><span class="p">,</span><span class="w"></span>
 130 <span class="w">                           </span><span class="n">pat</span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="p">,</span><span class="w"></span>
 131 <span class="w">                           </span><span class="n">var</span><span class="p">:</span><span class="w"> </span><span class="n">Var</span><span class="p">.</span><span class="n">t</span><span class="p">}</span><span class="w"></span>
 132 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">List</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 133 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Paren</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 134 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Or</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 135 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Record</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">flexible</span><span class="p">:</span><span class="w"> </span><span class="n">bool</span><span class="p">,</span><span class="w"></span>
 136 <span class="w">                          </span><span class="n">items</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">Record</span><span class="p">.</span><span class="n">Field</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">Region</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">Item</span><span class="p">.</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">vector</span><span class="p">}</span><span class="w"></span>
 137 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Tuple</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 138 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">fixop</span><span class="p">:</span><span class="w"> </span><span class="n">Fixop</span><span class="p">.</span><span class="n">t</span><span class="p">,</span><span class="w"></span>
 139 <span class="w">                       </span><span class="n">name</span><span class="p">:</span><span class="w"> </span><span class="n">Longvid</span><span class="p">.</span><span class="n">t</span><span class="p">}</span><span class="w"></span>
 140 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Vector</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 141 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Wild</span><span class="w"></span>
 142 </pre></div></div></div>
 143 <div class="paragraph"><p>Thus, AST nodes are cleanly separated from source locations.  By way
 144 of contrast, consider the approach taken by <a href="SMLNJ">SML/NJ</a> (and also
 145 by the <a href="CKitLibrary">CKit Library</a>).  Each datatype denoting a syntax
 146 phrase dedicates a special constructor for annotating source
 147 locations:</p></div>
 148 <div class="listingblock">
 149 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">pat</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">WildPat</span><span class="w">                             </span><span class="cm">(* empty pattern *)</span><span class="w"></span>
 150 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">AppPat</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">constr</span><span class="p">:</span><span class="n">pat</span><span class="p">,</span><span class="n">argument</span><span class="p">:</span><span class="n">pat</span><span class="p">}</span><span class="w"> </span><span class="cm">(* application *)</span><span class="w"></span>
 151 <span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">MarkPat</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">pat</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">region</span><span class="w">             </span><span class="cm">(* mark a pattern *)</span><span class="w"></span>
 152 </pre></div></div></div>
 153 <div class="paragraph"><p>The main drawback of this approach is that static type checking is not
 154 sufficient to guarantee that the AST emitted from the front-end is
 155 properly annotated.</p></div>
 156 </div>
 157 </div>
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