...

...

type R m = m -> Maybe (R m, [m])

I don't think there's an extension of Haskell with regular type unification. It's certainly possible, but there's an equivalent in standard Haskell:

newtype R m = MkR (m -> Maybe (R m, [m]))

It will possibly be more convenient to break the cycle in another place, where we will wrap and pattern match anyway:

type R m = m -> Response m data Response m | NotTaken | Taken (R m) [m]

Thanks everyone for responding so swiftly. The trick by Marcin really cleaned up my code, thanks! Cheers, l4t3r _________________________________________________________________ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail