Add 'state' to the MonadState class

As discussed here: http://www.haskell.org/pipermail/libraries/2011-September/016812.html

Change the Control.Monad.State.Class module in the "mtl" and "monad-tf" libraries, by adding a method to the MonadState class:

class MonadState s m | m -> s where
    ...
    state :: (s -> (a,s)) -> m a -- for mtl
    state :: (StateType m -> (a, StateType m)) -> m a -- for monads-tf
    state f = do s <- get
                 let (a,s') = f s
                 set s'
                 return a

And change the modify function to use 'state':

modify f = state $ \s -> ((),f s)

And add the appropriate instances:

instance (Monad m) => MonadState s (Lazy.StateT s m) where
    ...
    state f = Lazy.state

instance (Monad m) => MonadState s (Strict.StateT s m) where
    ...
    state f = Strict.state

instance (Monad m, Monoid w) => MonadState s (LazyRWS.RWST r w s m) where
    ...
    state f = LazyRWS.state

instance (Monad m, Monoid w) => MonadState s (StrictRWS.RWST r w s m) where
    ...
    state f = StrictRWS.state

The actual implementations should go into the "transformers" library:

state :: Monad m => (s -> (a,s)) -> StateT s m a
state f = StateT $ return . f

state :: (Monoid w, Monad m) => (s -> (a,s)) -> RWST r w s m a
state f = RWST $ \_ s -> let (a,s') = f a  in  return (a, s', mempty)

Note that there is already a function named 'state' in Control.Monad.State, which has 'm' restricted to 'Identity'. This more general function would be a replacement.

Trac metadata

Trac field	Value
Version	7.2.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture