No, it's behaving exactly as expected. If you omit UndecidableInstances the program is rejected. If you add that flag you are saying "you are allowed to diverge if I screw up". And indeed you wrote a looping type problem. I added some comments below that may help explain. Simon {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, UndecidableInstances #-} module X() where class C a b | a -> b where f :: a -> b newtype T a = T a instance (C a b, Eq b) => Eq (T a) where (==) = undefined g x = (undefined :: d -> d -> d -> ()) (T x) (f x) (undefined :: Eq e => e) -- f :: C a b => a -> b -- x :: a -- b ~ T a -- C a b -- b ~ e -- Eq e {- Hence need (C a (T a), Eq (T a)) Apply instance for Eq = (C a (T a), C a g, Eq g) Apply functional dependency: g ~ T a = (C a (T a), C a (T a), Eq (T a)) And now we are back where we started -} | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell- | users-bounces@haskell.org] On Behalf Of Roland Zumkeller | Sent: 29 October 2009 04:55 | To: glasgow-haskell-users@haskell.org | Subject: ghc hung by FunctionalDependencies/UndecidableInstances | | Hi, | | ghc seems to hang and eat memory when fed the following code: | | {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, | UndecidableInstances #-} | class C a b | a -> b where f :: a -> b | newtype T a = T a | instance (C a b, Eq b) => Eq (T a) where (==) = undefined | g x = (undefined :: a -> a -> a -> ()) (T x) (f x) (undefined :: Eq a => a) | | Is this a bug? | | Best, | | Roland | | -- | http://alacave.net/~roland/ | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users