doc/guide/src/Defunctorize.adoc

   1 Defunctorize
   2 ============
   3
   4 <:Defunctorize:> is a translation pass from the <:CoreML:>
   5 <:IntermediateLanguage:> to the <:XML:> <:IntermediateLanguage:>.
   6
   7 == Description ==
   8
   9 This pass converts a <:CoreML:> program to an <:XML:> program by
  10 performing:
  11
  12 * linearization
  13 * <:MatchCompile:>
  14 * polymorphic `val` dec expansion
  15 * `datatype` lifting (to the top-level)
  16
  17 == Implementation ==
  18
  19 * <!ViewGitFile(mlton,master,mlton/defunctorize/defunctorize.sig)>
  20 * <!ViewGitFile(mlton,master,mlton/defunctorize/defunctorize.fun)>
  21
  22 == Details and Notes ==
  23
  24 This pass is grossly misnamed and does not perform defunctorization.
  25
  26 === Datatype Lifting ===
  27
  28 This pass moves all `datatype` declarations to the top level.
  29
  30 <:StandardML:Standard ML> `datatype` declarations can contain type
  31 variables that are not bound in the declaration itself.  For example,
  32 the following program is valid.
  33 [source,sml]
  34 ----
  35 fun 'a f (x: 'a) =
  36    let
  37       datatype 'b t = T of 'a * 'b
  38       val y: int t = T (x, 1)
  39    in
  40       13
  41    end
  42 ----
  43
  44 Unfortunately, the `datatype` declaration can not be immediately moved
  45 to the top level, because that would leave `'a` free.
  46 [source,sml]
  47 ----
  48 datatype 'b t = T of 'a * 'b
  49 fun 'a f (x: 'a) =
  50    let
  51       val y: int t = T (x, 1)
  52    in
  53       13
  54    end
  55 ----
  56
  57 In order to safely move `datatype`s, this pass must close them, as
  58 well as add any free type variables as extra arguments to the type
  59 constructor.  For example, the above program would be translated to
  60 the following.
  61 [source,sml]
  62 ----
  63 datatype ('a, 'b) t = T of 'a * 'b
  64 fun 'a f (x: 'a) =
  65    let
  66       val y: ('a * int) t = T (x, 1)
  67    in
  68       13
  69    end
  70 ----
  71
  72 == Historical Notes ==
  73
  74 The <:Defunctorize:> pass originally eliminated
  75 <:StandardML:Standard ML> functors by duplicating their body at each
  76 application.  These duties have been adopted by the <:Elaborate:>
  77 pass.