David Roundy
My latest attemp (which won't compile with the HEAD ghc that I just compiled, probably because I haven't figured out the synatax for guards with indexed types is:
class WitnessMonad m where type W m :: * -> * -> * (>>=) :: (WitnessMonad m', WitnessMonad m'', w a b = W m', w b c = W m'', w a c = W m) => m' x -> (x -> m'' y) -> m y (>>) :: (WitnessMonad m', WitnessMonad m'', w a b = W m', w b c = W m'', w a c = W m) => m' x -> m'' y -> m y f >> g = f >>= const g return :: w a a = W m x => -> m x fail :: String -> m x
data Witness a b
instance Monad m => WitnessMonad m where W m = Witness () () (>>=) = Prelude.(>>=) (>>) = Prelude.(>>) return = Prelude.return fail = Prelude.fail
which I think is quite pretty. It allows the Monadlike object to have kind * -> *, while still allowing us to hide extra witness types inside and pull them out using the W function.
Did anyone with knowledge of Associated Types pursue this solution? It doesn't work with GHC head, and I can't really do anything about that. Mostly curiosity. Thanks pepe ---------------------------------------------------------------- Everything from here on is to convince GMane that, even if my message contains more quoted text than fresh text, it is a legitimate message and it should be ok to post it.