Kim-Ee Yeoh wrote:
How about foo :: (exists. m :: * -> *. forall a. a -> m a) -> (m Char, m Bool)
Thank you: I had actually thought about something like that. First, the exists above should actually span over the whole type, so it becomes a forall because (->) is contravariant on its 1st argument: foo :: forall m :: * -> * . (forall a. a -> m a) -> (m Char, m Bool) This seems to be Haskell+extensions, but note that m above is meant to be an arbitrary type-level function, and not a type constructor (in general). So, I am not surprised if undecidability issues arise in type checking/inference. :-) Another valid type for foo can be done AFAICS with intersection types: foo :: (Char -> a /\ Bool -> b) -> (a,b) But I can not comment about their inference, or usefulness in practice. Regards, Zun.