Make the treatment of equalities more uniform

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

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index 636ee0e..165018d 100644 (file)
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -670,9 +670,7 @@ tcSplitPhiTy ty = split ty ty []
   split orig_ty ty tvs | Just ty' <- tcView ty = split orig_ty ty' tvs
 
   split orig_ty (ForAllTy tv ty) ts
-        | isCoVar tv = split ty ty (eq_pred:ts)
-        where
-           PredTy eq_pred = tyVarKind tv
+        | isCoVar tv = split ty ty (coVarPred tv : ts)
   split orig_ty (FunTy arg res) ts 
        | Just p <- tcSplitPredTy_maybe arg = split res res (p:ts)
   split orig_ty ty             ts = (reverse ts, orig_ty)
@@ -1062,6 +1060,15 @@ exactTyVarsOfType is used by the type checker to figure out exactly
 which type variables are mentioned in a type.  It's also used in the
 smart-app checking code --- see TcExpr.tcIdApp
 
+On the other hand, consider a *top-level* definition
+       f = (\x -> x) :: T a -> T a
+If we don't abstract over 'a' it'll get fixed to GHC.Prim.Any, and then
+if we have an application like (f "x") we get a confusing error message 
+involving Any.  So the conclusion is this: when generalising
+  - at top level use tyVarsOfType
+  - in nested bindings use exactTyVarsOfType
+See Trac #1813 for example.
+
 \begin{code}
 exactTyVarsOfType :: TcType -> TyVarSet
 -- Find the free type variables (of any kind)