On Sat, Oct 08, 2011 at 04:28:34PM -0700, Captain Freako wrote:
Hi all,
I'm trying to use the State Monad to help implement a digital filter:
(a -> EitherT e (State FilterState) a) is definitely not monadic. There is an 'a' in a negative position (to the left of an odd number of arrows) so it is not even a Functor. But I think you want to have the 'a ->' part separate. I might do something like newtype FilterM e a = F { unFilterM :: EitherT e (State FilterState) a } deriving (Functor, Applicative, Monad, MonadState FilterState) type Filter e a b = a -> FilterM e b That is, a Filter is a function from a to b, which can abort with an error of type e and maintains a FilterState. Then you can chain Filters using (>=>): if f1 :: Filter e a b f2 :: Filter e b c then f1 >=> f2 :: Filter e a c is a Filter which does first f1 then f2. Alternatively, if you wanted to use an Arrow interface you could define import Control.Arrow type Filter e a b = Kleisli (FilterM e) a b -Brent