Tom Hawkins wrote:
I'm just starting to feel comfortable working inside IO; now I am trying to wrap my head around Control.Monad.State. Looking at the implementation, I came across some unfamiliar syntax...
class (Monad m) => MonadState s m | m -> s where
For a simple state monad the class MonadState is only important to supply the functions set, get and (indirectly) modify and gets. More important to note (and independent of MonadState) is: instance Monad (State s) This instance is not directly displayed by the haddock documentation, only (indirectly): instance MonadState s (State s)
With IO, I understand one your in it, you can't get out. I.e., any function that conducts IO must have IO _ in the type signature.
"IO" is kind of a hidden type constructor whereas newtype State s a = State { runState :: s -> (a, s) } is fully known. Maybe it helps to view the "type IO a" as "State RealWorld a" where "Realworld" is never exposed (and of course without "instance MonadState RealWorld IO"). Christian