[Haskell-beginners] Re: Trying to find an alternative StateMonad

[edgar klerks]

I think I already know, the monad is not a state monad, but a writer
monad. I have to implement the dsl in a state monad.

Sorry I was somewhat too fast sending to the list. But comments are of
course still welcome.

On 4/15/10, edgar klerks <edgar.klerks at gmail.com> wrote:
> Hi All,
>
> I wanted to write the statemonad in a different way, because I want to
> track mutations to the state. And also to be able to undo an done or
> redo an undone mutation. Therefore I wrote a small dsl with an
> evaluator, which finds out the current state.
>
> The problem is, I don't know how to create a get and put function for
> my monad. I have a function which evaluate the state and gives back
> the current State.
>
> Can someone help me a bit along? I have the feeling I do something
> wrong, but I am not sure what.
>
> With kind regards,
>
> Edgar
>
> {-# LANGUAGE GADTs,DeriveFunctor  #-}
> import Control.Monad
> import Data.Maybe
>
> data StateCmd a where
> 	Put :: a -> StateCmd a
> 	Undo :: StateCmd a
> 	Redo :: StateCmd a
> 		deriving (Show, Functor)
>
> newtype State s a = State { unState :: ([StateCmd s],a) }
> 	deriving Show
>
> joinState :: State s a -> State s b -> State s b
> joinState (State (xs, a)) (State (ys, b)) = State (xs ++ ys, b)
>
> instance Monad (State s) where
> 	return a = State $ ([], a)
> 	(>>=) = bindState
>
> -- m a -> ( a -> m b) -> m b
> bindState st@(State (_,a)) f = st `joinState` st'
> 	where st' = f a
>
> unPut :: StateCmd a -> Maybe a
> unPut (Put a) = Just a
> unPut _ = Nothing
>
> test = State $ ([Put 4, Undo, Redo, Undo, Put 5, Undo, Redo,  Put 6,
> Undo, Redo, Undo], ())
>
> getCurrent = fromJust.unPut.head.snd.(foldr current'
> ([],[])).reverse.fst.unState
> 		where current' x (ul,cl) = case x of
> 					Put n -> (ul, Put n : cl)
> 					Undo -> (head cl : ul, tail cl)
> 					Redo -> (tail ul, head ul : cl)
>


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