Add Note [Generating the in-scope set for a substitution]

[ghc-hetmet.git] / compiler / types / Type.lhs

diff --git a/compiler/types/Type.lhs b/compiler/types/Type.lhs
index 686bba8..db23582 100644 (file)
--- a/compiler/types/Type.lhs
+++ b/compiler/types/Type.lhs
@@ -55,7 +55,7 @@ module Type (
        splitTyConApp_maybe, splitTyConApp, 
         splitNewTyConApp_maybe, splitNewTyConApp,
 
-       repType, typePrimRep, coreView, tcView, kindView, rttiView,
+       repType, typePrimRep, coreView, tcView, kindView,
 
        mkForAllTy, mkForAllTys, splitForAllTy_maybe, splitForAllTys, 
        applyTy, applyTys, isForAllTy, dropForAlls,
@@ -188,16 +188,6 @@ tcView (TyConApp tc tys) | Just (tenv, rhs, tys') <- tcExpandTyCon_maybe tc tys
 tcView _                 = Nothing
 
 -----------------------------------------------
-rttiView :: Type -> Type
--- Same, but for the RTTI system, which cannot deal with predicates nor polymorphism
-rttiView (ForAllTy _ ty) = rttiView ty
-rttiView (FunTy PredTy{} ty) = rttiView ty
-rttiView ty@TyConApp{} | Just ty' <- coreView ty 
-                           = rttiView ty'
-rttiView (TyConApp tc tys) = mkTyConApp tc (map rttiView tys)
-rttiView ty = ty
-
------------------------------------------------
 {-# INLINE kindView #-}
 kindView :: Kind -> Maybe Kind
 -- C.f. coreView, tcView
@@ -1268,6 +1258,19 @@ extendTvSubstList (TvSubst in_scope env) tvs tys
 -- the types given; but it's just a thunk so with a bit of luck
 -- it'll never be evaluated
 
+-- Note [Generating the in-scope set for a substitution]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- If we want to substitute [a -> ty1, b -> ty2] I used to 
+-- think it was enough to generate an in-scope set that includes
+-- fv(ty1,ty2).  But that's not enough; we really should also take the
+-- free vars of the type we are substituting into!  Example:
+--     (forall b. (a,b,x)) [a -> List b]
+-- Then if we use the in-scope set {b}, there is a danger we will rename
+-- the forall'd variable to 'x' by mistake, getting this:
+--     (forall x. (List b, x, x)
+-- Urk!  This means looking at all the calls to mkOpenTvSubst....
+
+
 mkOpenTvSubst :: TvSubstEnv -> TvSubst
 mkOpenTvSubst env = TvSubst (mkInScopeSet (tyVarsOfTypes (varEnvElts env))) env