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+{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
+             FlexibleInstances,
+             OverlappingInstances, UndecidableInstances #-}
+
+-- Rather exotic example posted to Haskell mailing list 17 Oct 07
+-- It concerns context reduction and functional dependencies
+
+module FooModule where
+
+class Concrete a b | a -> b where
+	bar :: a -> String
+
+instance (Show a) => Concrete a b where
+	bar = error "urk"
+
+wib :: Concrete a b => a -> String
+wib x = bar x
+
+-- Uncommenting this solves the problem:
+-- instance Concrete Bool Bool
+
+{- This is a nice example of the trickiness of functional dependencies.
+Here's what is happening.
+
+Consider type inference for 'wib'.  GHC 6.6 figures out that the call
+of 'bar' gives rise to the constraint (Concrete p q), where x has type
+'p'.  Ah, but x must have type 'a', so the constraint is (Concrete a
+q).
+
+Now GHC tries to satisfy (Concrete a q) from (Concrete a b). If it
+applied improvement right away it'd succeed, but sadly it first looks
+at instances declarations.  Success: we can get (Concrete a q) from
+(Show a).  So it uses the instance decl and now we can't get (Show a)
+from (Concrete a b).
+
+
+OK,  found that in GHC 6.6, adding
+	instance Concrete Bool Bool
+fixed the problem. That's weird isn't it?  The reason is this. When GHC looks
+at the instance decls, it now sees *two* instance decls matching
+(Concrete a q), and so it declines for now to use either of them
+(since it's not clear which would be the right one).  Once it has
+finished with instance decls it tries improvement.  And, yes, it now
+sees that q=b, so all is well.
+
+You might say that GHC should use improvement more vigorously, and
+perhaps you'd be right.  And indeed the upcoming GHC 6.8 does exactly
+that.
+-}
+