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 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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