[project @ 2003-02-14 14:19:15 by simonpj]

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

diff --git a/ghc/compiler/typecheck/TcUnify.lhs b/ghc/compiler/typecheck/TcUnify.lhs
index fd22557..e4116e2 100644 (file)
--- a/ghc/compiler/typecheck/TcUnify.lhs
+++ b/ghc/compiler/typecheck/TcUnify.lhs
@@ -100,7 +100,8 @@ These two check for holes
 
 \begin{code}
 tcSubExp expected_ty offered_ty
-  = checkHole expected_ty offered_ty tcSub
+  = traceTc (text "tcSubExp" <+> (ppr expected_ty $$ ppr offered_ty))  `thenM_`
+    checkHole expected_ty offered_ty tcSub
 
 tcSubOff expected_ty offered_ty
   = checkHole offered_ty expected_ty (\ off exp -> tcSub exp off)
@@ -116,7 +117,8 @@ checkHole (TyVarTy tv) other_ty thing_inside
   = getTcTyVar tv      `thenM` \ maybe_ty ->
     case maybe_ty of
        Just ty -> thing_inside ty other_ty
-       Nothing -> putTcTyVar tv other_ty       `thenM_`
+       Nothing -> traceTc (text "checkHole" <+> ppr tv)        `thenM_`
+                  putTcTyVar tv other_ty       `thenM_`
                   returnM idCoercion
 
 checkHole ty other_ty thing_inside 
@@ -129,7 +131,7 @@ No holes expected now.  Add some error-check context info.
 tcSub expected_ty actual_ty
   = traceTc (text "tcSub" <+> details)         `thenM_`
     addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
-                 (tc_sub expected_ty expected_ty actual_ty actual_ty)
+               (tc_sub expected_ty expected_ty actual_ty actual_ty)
   where
     details = vcat [text "Expected:" <+> ppr expected_ty,
                    text "Actual:  " <+> ppr actual_ty]
@@ -199,7 +201,19 @@ tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
 --      when the arg/res is not a tau-type?
 -- NO!  e.g.   f :: ((forall a. a->a) -> Int) -> Int
 --     then   x = (f,f)
---     is perfectly fine!
+--     is perfectly fine, because we can instantiat f's type to a monotype
+--
+-- However, we get can get jolly unhelpful error messages.  
+--     e.g.    foo = id runST
+--
+--    Inferred type is less polymorphic than expected
+--     Quantified type variable `s' escapes
+--     Expected type: ST s a -> t
+--     Inferred type: (forall s1. ST s1 a) -> a
+--    In the first argument of `id', namely `runST'
+--    In a right-hand side of function `foo': id runST
+--
+-- I'm not quite sure what to do about this!
 
 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
   = ASSERT( not (isHoleTyVar tv) )