SETrans.hs

{-# LANGUAGE ScopedTypeVariables #-}

import Control.Monad.Identity
import Control.Monad.Except
import Control.Monad.State.Strict


type SET s e m = StateT s (ExceptT e m)
type EST e s m = ExceptT e (StateT s m)

type SE s e = StateT s (Except e)
type ES e s = ExceptT e (State s)

data St = St
data Err = Err

-- errors w/ rollback
runES :: forall e s a . ES e s a -> s -> (Either e a, s)
runES act s0 = runState (runExceptT act :: State s (Either e a)) s0

-- errors w/o rollback
runSE :: forall s e a . SE s e a -> s -> Either e (a,s)
runSE act s0 = runExcept (runStateT act s0)