doc/guide/src/ScopeInference.adoc

   1 ScopeInference
   2 ==============
   3
   4 Scope inference is an analysis/rewrite pass for the <:AST:>
   5 <:IntermediateLanguage:>, invoked from <:Elaborate:>.
   6
   7 == Description ==
   8
   9 This pass adds free type variables to the `val` or `fun`
  10 declaration where they are implicitly scoped.
  11
  12 == Implementation ==
  13
  14 <!ViewGitFile(mlton,master,mlton/elaborate/scope.sig)>
  15 <!ViewGitFile(mlton,master,mlton/elaborate/scope.fun)>
  16
  17 == Details and Notes ==
  18
  19 Scope inference determines for each type variable, the declaration
  20 where it is bound.  Scope inference is a direct implementation of the
  21 specification given in section 4.6 of the
  22 <:DefinitionOfStandardML: Definition>.  Recall that a free occurrence
  23 of a type variable `'a` in a declaration `d` is _unguarded_
  24 in `d` if `'a` is not part of a smaller declaration.  A type
  25 variable `'a` is implicitly scoped at `d` if `'a` is
  26 unguarded in `d` and `'a` does not occur unguarded in any
  27 declaration containing `d`.
  28
  29 The first pass of scope inference walks down the tree and renames all
  30 explicitly bound type variables in order to avoid name collisions.  It
  31 then walks up the tree and adds to each declaration the set of
  32 unguarded type variables occurring in that declaration.  At this
  33 point, if declaration `d` contains an unguarded type variable
  34 `'a` and the immediately containing declaration does not contain
  35 `'a`, then `'a` is implicitly scoped at `d`.  The final
  36 pass walks down the tree leaving a `'a` at the a declaration where
  37 it is scoped and removing it from all enclosed declarations.