X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=8b3724a921a5de5ef1161825fe2983f3c73bb1e7;hb=2026e951c4811e0aaf44212426c7048cf5e8d0e8;hp=e1424189285d4173798b8f451d37cc284f2112da;hpb=6e0c3f50e131f502577a61b09a339af295de9d23;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index e142418..8b3724a 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -86,6 +86,24 @@ import DynFlags		( DynFlags(ctxtStkDepth),
 	Notes on functional dependencies (a bug)
 	--------------------------------------
 
+Consider this:
+
+	class C a b | a -> b
+	class D a b | a -> b
+
+	instance D a b => C a b	-- Undecidable 
+				-- (Not sure if it's crucial to this eg)
+	f :: C a b => a -> Bool
+	f _ = True
+	
+	g :: C a b => a -> Bool
+	g = f
+
+Here f typechecks, but g does not!!  Reason: before doing improvement,
+we reduce the (C a b1) constraint from the call of f to (D a b1).
+
+Here is a more complicated example:
+
 | > class Foo a b | a->b
 | >
 | > class Bar a b | a->b
@@ -257,9 +275,9 @@ any other type variables.
 
 
 
-	--------------------------------------
-		Notes on ambiguity
-	--------------------------------------
+-------------------------------------
+	Note [Ambiguity]
+-------------------------------------
 
 It's very hard to be certain when a type is ambiguous.  Consider
 
@@ -2451,8 +2469,8 @@ addNoInstanceErrs mb_what givens dicts
 			        ptext SLIT("to the") <+> what] ]
 
     	fix2 | null instance_dicts = []
-	     | otherwise	   = [ ptext SLIT("add an instance declaration for")
-				       <+> pprDictsTheta instance_dicts ]
+	     | otherwise	   = [ sep [ptext SLIT("add an instance declaration for"),
+				            pprDictsTheta instance_dicts] ]
 	instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
 		-- Insts for which it is worth suggesting an adding an instance declaration
 		-- Exclude implicit parameters, and tyvar dicts