Max Bolingbroke wrote:
I don't actually know what the right name for this data type is, I just invented it and it seems to work:
-- (>>>) :: forall a b. t a b -> (forall c. t b c -> t a c) newtype Wotsit t a b = Wotsit { runWotsit :: forall c. t b c -> t a c }
There is of course no reason to prefer (>>>) to (<<<), so you can instead quantify over the first argument as opposed to second one: newtype Wotsit' t a b = Wotsit' { runWotsit' :: forall c. t c a -> t c b } liftWotsit' :: Category t => t a b -> Wotsit' t a b liftWotsit' t = Wotsit' ((<<<) t) lowerWotsit' :: Category t => Wotsit' t a b -> t a b lowerWotsit' t = runWotsit' t id instance Category (Wotsit' t) where id = Wotsit' id t1 . t2 = Wotsit' (runWotsit' t1 . runWotsit' t2) Twan