stack.hs

import Control.Monad.State

--
-- 普通に Stack
--

type Stack = [Int]

pop :: Stack -> (Int, Stack)
pop (x:xs) = (x, xs)

push :: Int -> Stack -> ((), Stack)
push a xs = ((), a:xs)

stackOp :: Stack -> (Int, Stack)
stackOp stack = let
  ((), newStack1) = push 3 stack
  (a, newStack2)  = pop newStack1
  in pop newStack2


-- 
-- Monad.State を使って Stack
--

type MonadStack = [Int]

mpop :: State MonadStack Int
mpop = state $ \(x:xs) -> (x, xs)

mpush :: Int -> State MonadStack ()
mpush a = state $ \xs -> ((), a:xs)

-- Monad を使うと綺麗に表現できる
monadStackOp :: State Stack Int
monadStackOp = do
  mpush 3
  a <- mpop
  if a == 5
    then mpush 5
    else do
      mpush 3
      mpush 8
  mpop

main = do
  print $ stackOp [5, 8, 2, 1] -- (5, [8, 2, 1])
  print $ runState monadStackOp [5, 8, 2, 1] -- (8, [3, 5, 8, 2, 1])