double_monad.hs

import Control.Monad.State
import Control.Monad.Trans

type StackState a = StateT [Integer] IO a
--type StackStateIO = StateT [Integer] IO ()

push :: Integer -> StackState ()
push a = do
    modify (a:)
    return ()

pop :: StackState Integer
pop = do
    item <- get
    modify (drop 1)
    return (head item)

crazyCombinedMonad :: StackState Integer
crazyCombinedMonad = do
    value <- pop
    liftIO (putStrLn ("popped value " ++ (show value)))
    return (123 :: Integer)

main :: IO ()
main = do
    value <- runStateT (crazyCombinedMonad) [6,4,3]
    putStrLn $ show $ value

-- Output:
-- popped value 6
-- (123,[4,3])