Re: [Haskell-cafe] Trouble using State Monad.

9 Oct 2011

      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  wrote:
...
On Sat, Oct 8, 2011 at 4:28 PM, Captain Freako 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