I just saw someone "misusing" haskell@haskell.org for bug reports. I am going to join especially - it is a funny one, - the bug hasn't been fixed since Sep. 2001, - it is not even put in the list of known bugs since then. --------------------------------------------------------------- The following code works with ghc but not with hugs (incl. Dec. 2001 release) import Monad class Monad m => C1 m x -- Monad m is implied by C1 but test diverges if constraint not present class (C1 m x) => C2 m x where c2 :: x -> m x instance Monad m => C1 m Bool instance C2 Maybe Bool where c2 = return test :: Maybe Bool test = c2 True --------------------------------------------------------------- So you see, C1 is a kind of derived from Monad. C2 is derived from C1 and hence implicitly from Monad. This is a kind of minimal example. Just attempt to evaluate "test". hugs will loop. There is a way to help hugs. Add Monad m as a further class constraint to C2. Then it will work. Please note both programs do type check, and this is in fact fine. More generally, I get these looping programs whenever I have non-trivial class hierarchies with three or more layers. I guess more people should have suffered from that? Ralf -- Dr.-Ing. Ralf Laemmel CWI & VU, Amsterdam, The Netherlands http://www.cwi.nl/~ralf/ http://www.cs.vu.nl/~ralf/