plp/haskell/haskell6.hs

module Haskell6 where

import Control.Monad.ST
--State Monad

----Stack
type Stack = [Int]

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

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

--Define Stackmanipulator
stackManip :: Stack -> (Int, Stack)
stackManip stack = let
        (a, newstack1) = pop stack
        (b, newstack2) = pop newstack1
        ((), newstack3) = push 100 newstack2
        (c, newstack4) = pop newstack3
     in pop newstack4

--Define the pop function using the state monad
popM :: State Stack Int
popM = do
        x:xs <- get
        put xs
        return x

--Define the push function using the state monad
pushM :: Int -> State Stack ()
push a = do
        xs <- get
        put (a:xs)
        return ()

--Redefine the stackManip using the monadic functions
stackManipM :: Stack State Int
stackManipM = do
        popM
        popM
        pushM 100
        popM
        popM