I wrote: > keep :: (forall c . (t -> c) -> u -> c) -> ((t1 -> t) -> b) -> (t1 -> > u) -> b > keep transform rec = \fn -> rec $ transform id . fn
Just to clarify - you don't really need the RankNTypes here, I just wrote it that way so you could see what I had been thinking, and to make it clear how the first parameter of keep takes transformers like drop'. But you could write it as keep :: ((t -> t) -> u -> t) -> ... and it would work just fine, because transformers like drop' would specialize nicely to what you need for keep. If you let GHC deduce the type of keep from its definition, GHC comes up with something else: keep :: ((a -> a) -> u -> t) -> ((t1 -> t) -> b) -> (t1 -> u) -> b That also works, but it's weird. It generalizes in a direction that we don't really need here, and thus obscures the meaning of what we're doing. Regards, Yitz _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
