Better error message for fundep conflict

diff --git a/compiler/types/Class.lhs b/compiler/types/Class.lhs
index fb7cab1..e7bda63 100644 (file)
--- a/compiler/types/Class.lhs
+++ b/compiler/types/Class.lhs
@@ -10,7 +10,7 @@ module Class (
        Class, ClassOpItem, 
        DefMeth (..),
 
-       FunDep, pprFundeps,
+       FunDep, pprFundeps, pprFunDep,
 
        mkClass, classTyVars, classArity,
        classKey, className, classATs, classSelIds, classTyCon, classMethods,
@@ -174,9 +174,9 @@ instance Outputable DefMeth where
 
 pprFundeps :: Outputable a => [FunDep a] -> SDoc
 pprFundeps []  = empty
-pprFundeps fds = hsep (ptext (sLit "|") : punctuate comma (map ppr_fd fds))
-              where
-                ppr_fd (us, vs) = hsep [interppSP us, ptext (sLit "->"), 
-                                        interppSP vs]
+pprFundeps fds = hsep (ptext (sLit "|") : punctuate comma (map pprFunDep fds))
+
+pprFunDep :: Outputable a => FunDep a -> SDoc
+pprFunDep (us, vs) = hsep [interppSP us, ptext (sLit "->"), interppSP vs]
 \end{code}
 

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 170304c..af7cfec 100644 (file)
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -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 _ _ _