Am Freitag, 19. Dezember 2008 00:17 schrieb Martijn van Steenbergen:
Good evening everyone,
My program reads:
module Boom where
import Control.Monad.State
type SucParser s = StateT [s] []
newtype WithUnit s a = WithUnit (SucParser s (a, ()))
foo :: SucParser s [s] foo = get
bar :: WithUnit s [s] bar = WithUnit get
The compiler complains:
Boom.hs:13:0: Couldn't match expected type `([s], ())' against inferred type `[s]' When using functional dependencies to combine MonadState s (StateT s m), arising from the instance declaration at <no location info> MonadState ([s], ()) (StateT [s] []), arising from a use of `get' at Boom.hs:13:15-17 When generalising the type(s) for `bar'
I'm wondering if I'm making a silly mistake or if there's something less trivial going on here. Could someone please explain the error and give a hint on how to fix it?
class MonadState s m | m -> s where get :: m s ... For SucParser, get :: StateT [s] [] [s]. To wrap it in WithUnit, it would need type StateT [s] [] (([s],()),[s]) Easy fix: bar :: WithUnit s [s] bar = WithUnit $ do s <- get return (s,()) Better define liftUnit :: SucParser s a -> WithUnit s a liftUnit m = WithUnit $ do a <- m return (a,()) and bar = liftUnit get Or make WithUnit more general (working with arbitrary monads) and give it a MonadTrans instance.
Thanks much. :-)
Martijn.