State Monad example

RPN.hs

module RPN where

import Control.Monad
import Control.Monad.State

type Stack = [Int]
type StackState = State Stack

push :: Int -> StackState ()
push n = modify (n:)

pop :: StackState Int
pop = do
    stack <- get
    modify (drop 1)
    return $ head stack

add :: StackState ()
add = do
    stack <- get
    when (length stack < 2) (error "Add need two oprand")
    let s = sum $ take 2 stack
    modify (\stack -> s : drop 2 stack)

begin :: StackState ()
begin = do
    modify (\s -> [])

end :: StackState Int
end = do
    stack <- get
    return $ head stack

evalRPN :: StackState a -> (a, Stack)
evalRPN exp = runState exp []

test = do
    begin
    push 1
    pop
    push 2
    push 3
    push 4
    end