doc/guide/localhost/XML

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>XML</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="XML">XML</a> is an <a href="IntermediateLanguage">IntermediateLanguage</a>, translated from <a href="CoreML">CoreML</a> by
  32 <a href="Defunctorize">Defunctorize</a>, optimized by <a href="XMLSimplify">XMLSimplify</a>, and translated by
  33 <a href="Monomorphise">Monomorphise</a> to <a href="SXML">SXML</a>.</p></div>
  34 </div>
  35 </div>
  36 <div class="sect1">
  37 <h2 id="_description">Description</h2>
  38 <div class="sectionbody">
  39 <div class="paragraph"><p><a href="XML">XML</a> is polymorphic, higher-order, with flat patterns.  Every
  40 <a href="XML">XML</a> expression is annotated with its type.  Polymorphic
  41 generalization is made explicit through type variables annotating
  42 <span class="monospaced">val</span> and <span class="monospaced">fun</span> declarations.  Polymorphic instantiation is made
  43 explicit by specifying type arguments at variable references.  <a href="XML">XML</a>
  44 patterns can not be nested and can not contain wildcards, constraints,
  45 flexible records, or layering.</p></div>
  46 </div>
  47 </div>
  48 <div class="sect1">
  49 <h2 id="_implementation">Implementation</h2>
  50 <div class="sectionbody">
  51 <div class="ulist"><ul>
  52 <li>
  53 <p>
  54 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/xml.sig"><span class="monospaced">xml.sig</span></a>
  55 </p>
  56 </li>
  57 <li>
  58 <p>
  59 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/xml.fun"><span class="monospaced">xml.fun</span></a>
  60 </p>
  61 </li>
  62 <li>
  63 <p>
  64 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/xml-tree.sig"><span class="monospaced">xml-tree.sig</span></a>
  65 </p>
  66 </li>
  67 <li>
  68 <p>
  69 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/xml-tree.fun"><span class="monospaced">xml-tree.fun</span></a>
  70 </p>
  71 </li>
  72 </ul></div>
  73 </div>
  74 </div>
  75 <div class="sect1">
  76 <h2 id="_type_checking">Type Checking</h2>
  77 <div class="sectionbody">
  78 <div class="paragraph"><p><a href="XML">XML</a> also has a type checker, used for debugging.  At present, the
  79 type checker is also the best specification of the type system of
  80 <a href="XML">XML</a>.  If you need more details, the type checker
  81 (<a href="https://github.com/MLton/mlton/blob/master/mlton/xml/type-check.sig"><span class="monospaced">type-check.sig</span></a>,
  82 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/type-check.fun"><span class="monospaced">type-check.fun</span></a>), is pretty short.</p></div>
  83 <div class="paragraph"><p>Since the type checker does not affect the output of the compiler
  84 (unless it reports an error), it can be turned off.  The type checker
  85 recursively descends the program, checking that the type annotating
  86 each node is the same as the type synthesized from the types of the
  87 expressions subnodes.</p></div>
  88 </div>
  89 </div>
  90 <div class="sect1">
  91 <h2 id="_details_and_notes">Details and Notes</h2>
  92 <div class="sectionbody">
  93 <div class="paragraph"><p><a href="XML">XML</a> uses the same atoms as <a href="CoreML">CoreML</a>, hence all identifiers
  94 (constructors, variables, etc.) are unique and can have properties
  95 attached to them.  Finally, <a href="XML">XML</a> has a simplifier (<a href="XMLShrink">XMLShrink</a>),
  96 which implements a reduction system.</p></div>
  97 <div class="sect2">
  98 <h3 id="_types">Types</h3>
  99 <div class="paragraph"><p><a href="XML">XML</a> types are either type variables or applications of n-ary type
 100 constructors.  There are many utility functions for constructing and
 101 destructing types involving built-in type constructors.</p></div>
 102 <div class="paragraph"><p>A type scheme binds list of type variables in a type.  The only
 103 interesting operation on type schemes is the application of a type
 104 scheme to a list of types, which performs a simultaneous substitution
 105 of the type arguments for the bound type variables of the scheme.  For
 106 the purposes of type checking, it is necessary to know the type scheme
 107 of variables, constructors, and primitives.  This is done by
 108 associating the scheme with the identifier using its property list.
 109 This approach is used instead of the more traditional environment
 110 approach for reasons of speed.</p></div>
 111 </div>
 112 <div class="sect2">
 113 <h3 id="_xmltree">XmlTree</h3>
 114 <div class="paragraph"><p>Before defining <span class="monospaced">XML</span>, the signature for language <a href="XML">XML</a>, we need to
 115 define an auxiliary signature <span class="monospaced">XML_TREE</span>, that contains the datatype
 116 declarations for the expression trees of <a href="XML">XML</a>.  This is done solely
 117 for the purpose of modularity&#8201;&#8212;&#8201;it allows the simplifier and type
 118 checker to be defined by separate functors (which take a structure
 119 matching <span class="monospaced">XML_TREE</span>).  Then, <span class="monospaced">Xml</span> is defined as the signature for a
 120 module containing the expression trees, the simplifier, and the type
 121 checker.</p></div>
 122 <div class="paragraph"><p>Both constructors and variables can have type schemes, hence both
 123 constructor and variable references specify the instance of the scheme
 124 at the point of references.  An instance is specified with a vector of
 125 types, which corresponds to the type variables in the scheme.</p></div>
 126 <div class="paragraph"><p><a href="XML">XML</a> patterns are flat (i.e. not nested).  A pattern is a
 127 constructor with an optional argument variable.  Patterns only occur
 128 in <span class="monospaced">case</span> expressions.  To evaluate a case expression, compare the
 129 test value sequentially against each pattern.  For the first pattern
 130 that matches, destruct the value if necessary to bind the pattern
 131 variables and evaluate the corresponding expression.  If no pattern
 132 matches, evaluate the default.  All patterns of a case statement are
 133 of the same variant of <span class="monospaced">Pat.t</span>, although this is not enforced by ML&#8217;s
 134 type system.  The type checker, however, does enforce this.  Because
 135 tuple patterns are irrefutable, there will only ever be one tuple
 136 pattern in a case expression and there will be no default.</p></div>
 137 <div class="paragraph"><p><a href="XML">XML</a> contains value, exception, and mutually recursive function
 138 declarations.  There are no free type variables in <a href="XML">XML</a>.  All type
 139 variables are explicitly bound at either a value or function
 140 declaration.  At some point in the future, exception declarations may
 141 go away, and exceptions may be represented with a single datatype
 142 containing a <span class="monospaced">unit ref</span> component to implement genericity.</p></div>
 143 <div class="paragraph"><p><a href="XML">XML</a> expressions are like those of <a href="CoreML">CoreML</a>, with the following
 144 exceptions.  There are no records expressions.  After type inference,
 145 all records (some of which may have originally been tuples in the
 146 source) are converted to tuples, because once flexible record patterns
 147 have been resolved, tuple labels are superfluous.  Tuple components
 148 are ordered based on the field ordering relation.  <a href="XML">XML</a> eta expands
 149 primitives and constructors so that there are always fully applied.
 150 Hence, the only kind of value of arrow type is a lambda.  This
 151 property is useful for flow analysis and later in code generation.</p></div>
 152 <div class="paragraph"><p>An <a href="XML">XML</a> program is a list of toplevel datatype declarations and a
 153 body expression.  Because datatype declarations are not generative,
 154 the defunctorizer can safely move them to toplevel.</p></div>
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