MLton 20210117

# ScopeInference

Scope inference is an analysis/rewrite pass for the AST IntermediateLanguage, invoked from Elaborate.

## Description

This pass adds free type variables to the val or fun declaration where they are implicitly scoped.

## Implementation

scope.sig scope.fun

## Details and Notes

Scope inference determines for each type variable, the declaration where it is bound. Scope inference is a direct implementation of the specification given in section 4.6 of the Definition. Recall that a free occurrence of a type variable 'a in a declaration d is unguarded in d if 'a is not part of a smaller declaration. A type variable 'a is implicitly scoped at d if 'a is unguarded in d and 'a does not occur unguarded in any declaration containing d.

The first pass of scope inference walks down the tree and renames all explicitly bound type variables in order to avoid name collisions. It then walks up the tree and adds to each declaration the set of unguarded type variables occurring in that declaration. At this point, if declaration d contains an unguarded type variable 'a and the immediately containing declaration does not contain 'a, then 'a is implicitly scoped at d. The final pass walks down the tree leaving a 'a at the a declaration where it is scoped and removing it from all enclosed declarations.