Hi David, Thanks for the reply. In trying to follow your advice, I arrived at this code: 17 newtype Filter e a = F { 18 runFilter :: EitherT e (State FilterState) a 19 } deriving (Monad, MonadState FilterState) 20 21 applyFilter :: Filter e a -> FilterState -> a -> (Either e a, FilterState) 22 applyFilter f s x = runState (runEitherT (runFilter f)) s which compiles ok, but I don't know where to put the `x' on the right side of the `=' in line 22. That is, I still don't understand how I'm supposed to feed the input, `x', into the filter. Thanks, -db On Sat, Oct 8, 2011 at 5:27 PM, David Barbour <dmbarbour@gmail.com> wrote:
On Sat, Oct 8, 2011 at 4:28 PM, Captain Freako <capn.freako@gmail.com>wrote:
17 newtype Filter e a = F { * 18 runFilter :: EitherT e (State FilterState) a ** * 19 } deriving (Monad, MonadState FilterState)
it compiles, but I can't figure out how I'd feed the input to the filter, in that case.
Input to Filter would be modeled with: 'a -> Filter e b'.
I would rename your 'runFilter' to 'unFilter', then define a function 'runFilter' accepting an initial FilterState.
Regards,
- db