Better error message for fundep conflict

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

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 69533dc..af7cfec 100644 (file)
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -203,7 +203,7 @@ type Equation = (TyVarSet, [(Type, Type)])
 pprEquation :: Equation -> SDoc
 pprEquation (qtvs, pairs) 
   = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)),
-         nest 2 (vcat [ ppr t1 <+> ptext (sLit ":=:") <+> ppr t2 | (t1,t2) <- pairs])]
+         nest 2 (vcat [ ppr t1 <+> ptext (sLit "~") <+> ppr t2 | (t1,t2) <- pairs])]
 \end{code}
 
 Given a bunch of predicates that must hold, such as
@@ -294,8 +294,9 @@ improveOne inst_env pred@(ClassP cls tys, _) preds
        , not (instanceCantMatch inst_tcs trimmed_tcs)
        , eqn <- checkClsFD qtvs fd cls_tvs tys_inst tys
        , let p_inst = (mkClassPred cls tys_inst, 
-                       ptext (sLit "arising from the instance declaration at")
-                       <+> ppr (getSrcLoc ispec))
+                       sep [ ptext (sLit "arising from the dependency") <+> quotes (pprFunDep fd)
+                           , ptext (sLit "in the instance declaration at") 
+                                 <+> ppr (getSrcLoc ispec)])
        ]
 
 improveOne _ _ _
@@ -320,7 +321,7 @@ checkClsFD qtvs fd clas_tvs tys1 tys2
 --                 tys2 = [Maybe t1, t2]
 --
 -- We can instantiate x to t1, and then we want to force
---     (Tree x) [t1/x]  :=:   t2
+--     (Tree x) [t1/x]  ~   t2
 --
 -- This function is also used when matching two Insts (rather than an Inst
 -- against an instance decl. In that case, qtvs is empty, and we are doing
@@ -494,16 +495,16 @@ badFunDeps cls_insts clas ins_tv_set ins_tys
 
 trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name]
 -- Computing rough_tcs for a particular fundep
---     class C a b c | a -> b where ... 
+--     class C a b c | a -> b where ...
 -- For each instance .... => C ta tb tc
--- we want to match only on the types ta, tc; so our
+-- we want to match only on the type ta; so our
 -- rough-match thing must similarly be filtered.  
--- Hence, we Nothing-ise the tb type right here
-trimRoughMatchTcs clas_tvs (_,rtvs) mb_tcs
+-- Hence, we Nothing-ise the tb and tc types right here
+trimRoughMatchTcs clas_tvs (ltvs, _) mb_tcs
   = zipWith select clas_tvs mb_tcs
   where
-    select clas_tv mb_tc | clas_tv `elem` rtvs = Nothing
-                        | otherwise           = mb_tc
+    select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc
+                         | otherwise           = Nothing
 \end{code}