Loosen the conditions for -XUndecidableInstances; fixes Trac #4200

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

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index a3484a9..dd91b06 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -57,6 +57,7 @@ module TcMType (
   zonkTcType, zonkTcTypes, zonkTcThetaType,
   zonkTcKindToKind, zonkTcKind, 
   zonkImplication, zonkWanted, zonkEvVar, zonkWantedEvVar,
+  zonkTcTypeAndSubst,
   tcGetGlobalTyVars, 
 
   readKindVar, writeKindVar
@@ -485,25 +486,23 @@ zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
 -----------------  Types
 
 zonkTcTypeCarefully :: TcType -> TcM TcType
+-- Do not zonk type variables free in the environment
 zonkTcTypeCarefully ty
   = do { env_tvs <- tcGetGlobalTyVars
-       ; zonkType (zonkTcTyVarCarefully env_tvs) ty }
-
-
-zonkTcTyVarCarefully :: TcTyVarSet -> TcTyVar -> TcM TcType
--- Do not zonk type variables free in the environment
-zonkTcTyVarCarefully env_tvs tv 
-  | tv `elemVarSet` env_tvs 
-  = return (TyVarTy tv)
-  | otherwise
-  = ASSERT( isTcTyVar tv )
-    case tcTyVarDetails tv of
-      SkolemTv {}  -> return (TyVarTy tv)
-      FlatSkol ty  -> zonkType (zonkTcTyVarCarefully env_tvs) ty
-      MetaTv _ ref -> do { cts <- readMutVar ref
-                        ; case cts of    
-                            Flexi       -> return (TyVarTy tv)
-                            Indirect ty -> zonkType (zonkTcTyVarCarefully env_tvs) ty }
+       ; zonkType (zonk_tv env_tvs) ty }
+  where
+    zonk_tv env_tvs tv
+      | tv `elemVarSet` env_tvs 
+      = return (TyVarTy tv)
+      | otherwise
+      = ASSERT( isTcTyVar tv )
+       case tcTyVarDetails tv of
+         SkolemTv {}  -> return (TyVarTy tv)
+         FlatSkol ty  -> zonkType (zonk_tv env_tvs) ty
+         MetaTv _ ref -> do { cts <- readMutVar ref
+                            ; case cts of    
+                                Flexi       -> return (TyVarTy tv)
+                                Indirect ty -> zonkType (zonk_tv env_tvs) ty }
 
 zonkTcType :: TcType -> TcM TcType
 -- Simply look through all Flexis
@@ -521,6 +520,23 @@ zonkTcTyVar tv
                             Flexi       -> return (TyVarTy tv)
                             Indirect ty -> zonkTcType ty }
 
+zonkTcTypeAndSubst :: TvSubst -> TcType -> TcM TcType
+-- Zonk, and simultaneously apply a non-necessarily-idempotent substitution
+zonkTcTypeAndSubst subst ty = zonkType zonk_tv ty
+  where
+    zonk_tv tv 
+      = case tcTyVarDetails tv of
+         SkolemTv {}  -> return (TyVarTy tv)
+         FlatSkol ty  -> zonkType zonk_tv ty
+         MetaTv _ ref -> do { cts <- readMutVar ref
+                            ; case cts of    
+                                Flexi       -> zonk_flexi tv
+                                Indirect ty -> zonkType zonk_tv ty }
+    zonk_flexi tv
+      = case lookupTyVar subst tv of
+          Just ty -> zonkType zonk_tv ty
+          Nothing -> return (TyVarTy tv)
+
 zonkTcTypes :: [TcType] -> TcM [TcType]
 zonkTcTypes tys = mapM zonkTcType tys
 
@@ -1616,14 +1632,12 @@ sizeTypes xs               = sum (map sizeType xs)
 
 -- Size of a predicate
 --
--- Equalities are a special case.  The equality itself doesn't contribute to the
--- size and as we do not count class predicates, we have to start with one less.
--- This is easy to see considering that, given
---   class C a b | a -> b
---   type family F a
--- constraints (C a b) and (F a ~ b) are equivalent in size.
+-- We are considering whether *class* constraints terminate
+-- Once we get into an implicit parameter or equality we
+-- can't get back to a class constraint, so it's safe
+-- to say "size 0".  See Trac #4200.
 sizePred :: PredType -> Int
 sizePred (ClassP _ tys')   = sizeTypes tys'
-sizePred (IParam _ ty)     = sizeType ty
-sizePred (EqPred ty1 ty2)  = sizeType ty1 + sizeType ty2 - 1
+sizePred (IParam {})       = 0
+sizePred (EqPred {})       = 0
 \end{code}