HCoop / hcoop / debian / mlton.git / blame
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| 6 | <title>MatchCompile</title>\r | |
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| 19 | <body class="article">\r | |
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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>MatchCompile</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 href="MatchCompile">MatchCompile</a> is a translation pass, agnostic in the\r | |
| 32 | <a href="IntermediateLanguage">IntermediateLanguage</a>s between which it translates.</p></div>\r | |
| 33 | </div>\r | |
| 34 | </div>\r | |
| 35 | <div class="sect1">\r | |
| 36 | <h2 id="_description">Description</h2>\r | |
| 37 | <div class="sectionbody">\r | |
| 38 | <div class="paragraph"><p><a href="MatchCompilation">Match compilation</a> converts a case expression with\r | |
| 39 | nested patterns into a case expression with flat patterns.</p></div>\r | |
| 40 | </div>\r | |
| 41 | </div>\r | |
| 42 | <div class="sect1">\r | |
| 43 | <h2 id="_implementation">Implementation</h2>\r | |
| 44 | <div class="sectionbody">\r | |
| 45 | <div class="ulist"><ul>\r | |
| 46 | <li>\r | |
| 47 | <p>\r | |
| 48 | <a href="https://github.com/MLton/mlton/blob/master/mlton/match-compile/match-compile.sig"><span class="monospaced">match-compile.sig</span></a>\r | |
| 49 | </p>\r | |
| 50 | </li>\r | |
| 51 | <li>\r | |
| 52 | <p>\r | |
| 53 | <a href="https://github.com/MLton/mlton/blob/master/mlton/match-compile/match-compile.fun"><span class="monospaced">match-compile.fun</span></a>\r | |
| 54 | </p>\r | |
| 55 | </li>\r | |
| 56 | </ul></div>\r | |
| 57 | </div>\r | |
| 58 | </div>\r | |
| 59 | <div class="sect1">\r | |
| 60 | <h2 id="_details_and_notes">Details and Notes</h2>\r | |
| 61 | <div class="sectionbody">\r | |
| 62 | <div class="listingblock">\r | |
| 63 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">matchCompile</span><span class="p">:</span><span class="w"></span>\r | |
| 64 | <span class="w"> </span><span class="p">{</span><span class="n">caseType</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="p">,</span><span class="w"> </span><span class="cm">(* type of entire expression *)</span><span class="w"></span>\r | |
| 65 | <span class="w"> </span><span class="n">cases</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">NestedPat</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="p">((</span><span class="n">Var</span><span class="p">.</span><span class="n">t</span><span class="w"> </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="p">-></span><span class="w"> </span><span class="n">Exp</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>\r | |
| 66 | <span class="w"> </span><span class="n">conTycon</span><span class="p">:</span><span class="w"> </span><span class="n">Con</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">Tycon</span><span class="p">.</span><span class="n">t</span><span class="p">,</span><span class="w"></span>\r | |
| 67 | <span class="w"> </span><span class="n">region</span><span class="p">:</span><span class="w"> </span><span class="n">Region</span><span class="p">.</span><span class="n">t</span><span class="p">,</span><span class="w"></span>\r | |
| 68 | <span class="w"> </span><span class="n">test</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>\r | |
| 69 | <span class="w"> </span><span class="n">testType</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="p">,</span><span class="w"></span>\r | |
| 70 | <span class="w"> </span><span class="n">tyconCons</span><span class="p">:</span><span class="w"> </span><span class="n">Tycon</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">{</span><span class="n">con</span><span class="p">:</span><span class="w"> </span><span class="n">Con</span><span class="p">.</span><span class="n">t</span><span class="p">,</span><span class="w"> </span><span class="n">hasArg</span><span class="p">:</span><span class="w"> </span><span class="n">bool</span><span class="p">}</span><span class="w"> </span><span class="n">vector</span><span class="p">}</span><span class="w"></span>\r | |
| 71 | <span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">Exp</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="p">(</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">Layout</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="p">{</span><span class="n">isOnlyExns</span><span class="p">:</span><span class="w"> </span><span class="n">bool</span><span class="p">})</span><span class="w"> </span><span class="n">vector</span><span class="p">)</span><span class="w"> </span><span class="n">vector</span><span class="p">)</span><span class="w"></span>\r | |
| 72 | </pre></div></div></div>\r | |
| 73 | <div class="paragraph"><p><span class="monospaced">matchCompile</span> is complicated by the desire for modularity between the\r | |
| 74 | match compiler and its caller. Its caller is responsible for building\r | |
| 75 | the right hand side of a rule <span class="monospaced">p => e</span>. On the other hand, the match\r | |
| 76 | compiler is responsible for destructing the test and binding new\r | |
| 77 | variables to the components. In order to connect the new variables\r | |
| 78 | created by the match compiler with the variables in the pattern <span class="monospaced">p</span>,\r | |
| 79 | the match compiler passes an environment back to its caller that maps\r | |
| 80 | each variable in <span class="monospaced">p</span> to the corresponding variable introduced by the\r | |
| 81 | match compiler.</p></div>\r | |
| 82 | <div class="paragraph"><p>The match compiler builds a tree of n-way case expressions by working\r | |
| 83 | from outside to inside and left to right in the patterns. For example,</p></div>\r | |
| 84 | <div class="listingblock">\r | |
| 85 | <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 86 | <span class="w"> </span><span class="p">(_,</span><span class="w"> </span><span class="n">C1</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">e1</span><span class="w"></span>\r | |
| 87 | <span class="p">|</span><span class="w"> </span><span class="p">(</span><span class="n">C2</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">C3</span><span class="w"> </span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">e2</span><span class="w"></span>\r | |
| 88 | </pre></div></div></div>\r | |
| 89 | <div class="paragraph"><p>is translated to</p></div>\r | |
| 90 | <div class="listingblock">\r | |
| 91 | <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"></span>\r | |
| 92 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">f1</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">e1</span><span class="w"></span>\r | |
| 93 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">f2</span><span class="w"> </span><span class="p">(</span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>\r | |
| 94 | <span class="k">in</span><span class="w"></span>\r | |
| 95 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 96 | <span class="w"> </span><span class="p">(</span><span class="n">x1</span><span class="p">,</span><span class="w"> </span><span class="n">x2</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
| 97 | <span class="w"> </span><span class="p">(</span><span class="k">case</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 98 | <span class="w"> </span><span class="n">C2</span><span class="w"> </span><span class="n">b'</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="k">case</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 99 | <span class="w"> </span><span class="n">C1</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">f1</span><span class="w"> </span><span class="n">a'</span><span class="w"></span>\r | |
| 100 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">C3</span><span class="w"> </span><span class="n">c'</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">f2</span><span class="p">(</span><span class="n">b'</span><span class="p">,</span><span class="n">c'</span><span class="p">)</span><span class="w"></span>\r | |
| 101 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">Match</span><span class="p">)</span><span class="w"></span>\r | |
| 102 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="k">case</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 103 | <span class="w"> </span><span class="n">C1</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">f1</span><span class="w"> </span><span class="n">a_</span><span class="w"></span>\r | |
| 104 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">Match</span><span class="p">))</span><span class="w"></span>\r | |
| 105 | <span class="k">end</span><span class="w"></span>\r | |
| 106 | </pre></div></div></div>\r | |
| 107 | <div class="paragraph"><p>Here you can see the necessity of abstracting out the ride hand sides\r | |
| 108 | of the cases in order to avoid code duplication. Right hand sides are\r | |
| 109 | always abstracted. The simplifier cleans things up. You can also see\r | |
| 110 | the new (primed) variables introduced by the match compiler and how\r | |
| 111 | the renaming works. Finally, you can see how the match compiler\r | |
| 112 | introduces the necessary default clauses in order to make a match\r | |
| 113 | exhaustive, i.e. cover all the cases.</p></div>\r | |
| 114 | <div class="paragraph"><p>The match compiler uses <span class="monospaced">numCons</span> and <span class="monospaced">tyconCons</span> to determine\r | |
| 115 | the exhaustivity of matches against constructors.</p></div>\r | |
| 116 | </div>\r | |
| 117 | </div>\r | |
| 118 | </div>\r | |
| 119 | <div id="footnotes"><hr></div>\r | |
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