Sequence_State_Monad_Example.hs

-- sequence a state monad
import Control.Monad.Trans.State

-- Example 1
let y =  do { z <- get; let a = (Prelude.last z) + 1 in put (z Prelude.++ [a]);return $ (Prelude.last z) + 1} :: State [Int] Int
runState (sequenceA [y,y,y]) $ [1]

-- Example: 2
type X = State (M.Map String Int) Int
let y s s1 =  do { z <- get; let a = z M.! s + 1 in put (M.insert s1 a z) ; let a = z M.! s + 1 in return $ a } :: X
runState (sequenceA [(y "a" "b"), (y "b" "c")]) $ M.fromList [("a", 10)]
-- > ([11,12],fromList [("a",10),("b",11),("c",12)])