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
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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