Reorganise TcSimplify (again); FIX Trac #1919

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

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index e96fdd4..625f855 100644 (file)
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -457,7 +457,7 @@ pprSkolTvBinding tv
     ppr_details (MetaTv (SigTv info) _) = ppr_skol info
     ppr_details (SkolemTv info)                = ppr_skol info
 
-    ppr_skol UnkSkol       = empty     -- Unhelpful; omit
+    ppr_skol UnkSkol       = ptext SLIT("is an unknown type variable") -- Unhelpful
     ppr_skol RuntimeUnkSkol = ptext SLIT("is an unknown runtime type")
     ppr_skol info           = sep [ptext SLIT("is a rigid type variable bound by"),
                                   sep [pprSkolInfo info, 
@@ -1062,6 +1062,15 @@ 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)