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<a href="./Home">MLton 20130715</a>
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<h1>AST</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="AST">AST</a> is the <a href="IntermediateLanguage">IntermediateLanguage</a> produced by the <a href="FrontEnd">FrontEnd</a>
and translated by <a href="Elaborate">Elaborate</a> to <a href="CoreML">CoreML</a>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>The abstract syntax tree produced by the <a href="FrontEnd">FrontEnd</a>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_implementation">Implementation</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-programs.sig"><span class="monospaced">ast-programs.sig</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-programs.fun"><span class="monospaced">ast-programs.fun</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-modules.sig"><span class="monospaced">ast-modules.sig</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-modules.fun"><span class="monospaced">ast-modules.fun</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-core.sig"><span class="monospaced">ast-core.sig</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ast/ast-core.fun"><span class="monospaced">ast-core.fun</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/tree/master/mlton/ast"><span class="monospaced">ast</span></a>
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_type_checking">Type Checking</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <a href="AST">AST</a> <a href="IntermediateLanguage">IntermediateLanguage</a> has no independent type
checker. Type inference is performed on an AST program as part of
<a href="Elaborate">Elaborate</a>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_details_and_notes">Details and Notes</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_source_locations">Source locations</h3>
<div class="paragraph"><p>MLton makes use of a relatively clean method for annotating the
abstract syntax tree with source location information.  Every source
program phrase is "wrapped" with the <span class="monospaced">WRAPPED</span> interface:</p></div>
<div class="listingblock">
<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>
<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">node&#39;</span><span class="w"></span>
<span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">obj</span><span class="w"></span>

<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>
<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>
<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>
<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>
<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>
<span class="w">   </span><span class="k">end</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>The key idea is that <span class="monospaced">node'</span> is the type of an unannotated syntax
phrase and <span class="monospaced">obj</span> is the type of its annotated counterpart. In the
implementation, every <span class="monospaced">node'</span> is annotated with a <span class="monospaced">Region.t</span>
(<a href="https://github.com/MLton/mlton/blob/master/mlton/control/region.sig"><span class="monospaced">region.sig</span></a>,
<a href="https://github.com/MLton/mlton/blob/master/mlton/control/region.sml"><span class="monospaced">region.sml</span></a>), which describes the
syntax phrase&#8217;s left source position and right source position, where
<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>,
<a href="https://github.com/MLton/mlton/blob/master/mlton/control/source-pos.sml"><span class="monospaced">source-pos.sml</span></a>) denotes a
particular file, line, and column.  A typical use of the <span class="monospaced">WRAPPED</span>
interface is illustrated by the following code:</p></div>
<div class="listingblock">
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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">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>
<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>
<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>
<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>
<span class="w">             </span><span class="p">|</span><span class="w"> </span><span class="n">Wild</span><span class="w"></span>

<span class="w">            </span><span class="k">include</span><span class="w"> </span><span class="n">WRAPPED</span><span class="w"> </span><span class="k">sharing</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">node&#39;</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">node</span><span class="w"></span>
<span class="w">                            </span><span class="k">sharing</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">obj</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>Thus, AST nodes are cleanly separated from source locations.  By way
of contrast, consider the approach taken by <a href="SMLNJ">SML/NJ</a> (and also
by the <a href="CKitLibrary">CKit Library</a>).  Each datatype denoting a syntax
phrase dedicates a special constructor for annotating source
locations:</p></div>
<div class="listingblock">
<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>
<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>
<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>
</pre></div></div></div>
<div class="paragraph"><p>The main drawback of this approach is that static type checking is not
sufficient to guarantee that the AST emitted from the front-end is
properly annotated.</p></div>
</div>
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