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.fre...@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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> Haskell-Cafe@haskell.org
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>
>
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