stack.hs

import Control.Monad.State

type Stack = [Int]

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

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

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

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pop' :: State Stack Int
pop' = state $ \(x:xs) -> (x,xs)

push' :: Int -> State Stack ()
push' a = state $ \xs -> ((), a:xs)

someStack' :: State Stack Int
someStack' = do
  push' 3
  a <- pop'
  pop'