[project @ 2002-09-17 13:00:14 by simonpj]

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

diff --git a/ghc/compiler/types/Type.lhs b/ghc/compiler/types/Type.lhs
index 93a421a..95f22c2 100644 (file)
--- a/ghc/compiler/types/Type.lhs
+++ b/ghc/compiler/types/Type.lhs
@@ -602,12 +602,27 @@ tyVarsOfType :: Type -> TyVarSet
 tyVarsOfType (TyVarTy tv)              = unitVarSet tv
 tyVarsOfType (TyConApp tycon tys)      = tyVarsOfTypes tys
 tyVarsOfType (NoteTy (FTVNote tvs) ty2) = tvs
-tyVarsOfType (NoteTy (SynNote ty1) ty2)        = tyVarsOfType ty1
+tyVarsOfType (NoteTy (SynNote ty1) ty2)        = tyVarsOfType ty2      -- See note [Syn] below
 tyVarsOfType (SourceTy sty)            = tyVarsOfSourceType sty
 tyVarsOfType (FunTy arg res)           = tyVarsOfType arg `unionVarSet` tyVarsOfType res
 tyVarsOfType (AppTy fun arg)           = tyVarsOfType fun `unionVarSet` tyVarsOfType arg
 tyVarsOfType (ForAllTy tyvar ty)       = tyVarsOfType ty `minusVarSet` unitVarSet tyvar
 
+--                     Note [Syn]
+-- Consider
+--     type T a = Int
+-- What are the free tyvars of (T x)?  Empty, of course!  
+-- Here's the example that Ralf Laemmel showed me:
+--     foo :: (forall a. C u a -> C u a) -> u
+--     mappend :: Monoid u => u -> u -> u
+--
+--     bar :: Monoid u => u
+--     bar = foo (\t -> t `mappend` t)
+-- We have to generalise at the arg to f, and we don't
+-- want to capture the constraint (Monad (C u a)) because
+-- it appears to mention a.  Pretty silly, but it was useful to him.
+
+
 tyVarsOfTypes :: [Type] -> TyVarSet
 tyVarsOfTypes tys = foldr (unionVarSet.tyVarsOfType) emptyVarSet tys
 
@@ -628,8 +643,6 @@ addFreeTyVars ty@(NoteTy (FTVNote _) _)      = ty
 addFreeTyVars ty                            = NoteTy (FTVNote (tyVarsOfType ty)) ty
 \end{code}
 
-
-
 %************************************************************************
 %*                                                                     *
 \subsection{TidyType}