Your filter type isn't a Monad.

In particular

bind :: (a -> EitherT e (State FilterState) a) -> (a -> b -> EitherT e (State FilterState) b) -> b -> EitherT e (State FilterState) b

can't be implemented, as you have no place to grab an 'a' to pass to the initial computation.

If you fix the input type, you can do

newtype Filter r e a = F {
    runFilter :: r -> EitherT e (State FilterState) a
}

which is isomorphic to

newtype Filter r e a = F {
    runFilter :: ReaderT r (EitherT e (State FilterState)) a
}

which newtype deriving will be able to deal with easily.

  -- ryan


On Sat, Oct 8, 2011 at 4:28 PM, Captain Freako <capn.freako@gmail.com> wrote:
Hi all,

I'm trying to use the State Monad to help implement a digital filter:

 17 newtype Filter e a = F {
 18     runFilter :: a -> EitherT e (State FilterState) a
 19   } deriving (Monad, MonadState FilterState)

but I'm getting these compiler errors:

Filter.hs:19:14:
    Can't make a derived instance of `Monad (Filter e)'
      (even with cunning newtype deriving):
      cannot eta-reduce the representation type enough
    In the newtype declaration for `Filter'

Filter.hs:19:21:
    Can't make a derived instance of
      `MonadState FilterState (Filter e)'
      (even with cunning newtype deriving):
      cannot eta-reduce the representation type enough
    In the newtype declaration for `Filter'

If I change the code to this:

 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.

In comparing this to the tricks used in constructing the State Monad based version of the `Parser' type,
I notice that Parser gets around this issue, by having the input (i.e. - input stream) be a part of the initial state,
but I'm not sure that's appropriate for a digital filter.

Thanks,
-db

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