#5714: Add 'state' to the MonadState class
------------------------------+---------------------------------------------
Reporter: twanvl | Owner:
Type: task | Status: new
Priority: normal | Component: libraries (other)
Version: 7.2.1 | Keywords: mtl, monads-tf
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5714>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
