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