From dc667ab52ab208427094e497ece94132c57db8f2 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Mon, 1 Dec 2008 16:28:45 +0000
Subject: [PATCH] Better error message for fundep conflict

---
 compiler/types/Class.lhs   |   10 +++++-----
 compiler/types/FunDeps.lhs |    5 +++--
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/compiler/types/Class.lhs b/compiler/types/Class.lhs
index fb7cab1..e7bda63 100644
--- 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
--- 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 _ _ _
-- 
1.7.10.4