Use implication constraints to improve type inference

[ghc-hetmet.git] / compiler / typecheck / TcGadt.lhs

diff --git a/compiler/typecheck/TcGadt.lhs b/compiler/typecheck/TcGadt.lhs
index 4c1f70e..987a4c6 100644 (file)
--- a/compiler/typecheck/TcGadt.lhs
+++ b/compiler/typecheck/TcGadt.lhs
@@ -11,8 +11,9 @@
 
 \begin{code}
 module TcGadt (
-       Refinement, emptyRefinement, gadtRefine, 
-       refineType, refineResType,
+       Refinement, emptyRefinement, isEmptyRefinement, 
+       gadtRefine, 
+       refineType, refinePred, refineResType,
        dataConCanMatch,
        tcUnifyTys, BindFlag(..)
   ) where
@@ -22,6 +23,7 @@ module TcGadt (
 import HsSyn
 import Coercion
 import Type
+
 import TypeRep
 import DataCon
 import Var
@@ -61,6 +63,8 @@ instance Outputable Refinement where
 emptyRefinement :: Refinement
 emptyRefinement = (Reft emptyInScopeSet emptyVarEnv)
 
+isEmptyRefinement :: Refinement -> Bool
+isEmptyRefinement (Reft _ env) = isEmptyVarEnv env
 
 refineType :: Refinement -> Type -> Maybe (Coercion, Type)
 -- Apply the refinement to the type.
@@ -77,6 +81,17 @@ refineType (Reft in_scope env) ty
     tv_subst = mkTvSubst in_scope (mapVarEnv snd env)
     co_subst = mkTvSubst in_scope (mapVarEnv fst env)
  
+refinePred :: Refinement -> PredType -> Maybe (Coercion, PredType)
+refinePred (Reft in_scope env) pred
+  | not (isEmptyVarEnv env),           -- Common case
+    any (`elemVarEnv` env) (varSetElems (tyVarsOfPred pred))
+  = Just (mkPredTy (substPred co_subst pred), substPred tv_subst pred)
+  | otherwise
+  = Nothing    -- The type doesn't mention any refined type variables
+  where
+    tv_subst = mkTvSubst in_scope (mapVarEnv snd env)
+    co_subst = mkTvSubst in_scope (mapVarEnv fst env)
+ 
 refineResType :: Refinement -> Type -> (HsWrapper, Type)
 -- Like refineType, but returns the 'sym' coercion
 -- If (refineResType r ty) = (co, ty')