stackMachine.hs

import Prelude hiding (lex)

import Data.Char

import Data.Foldable (sequenceA_)

import Control.Monad.State

data Token = Num Integer
           | Op Char deriving (Show, Eq)

lex :: String -> [Token]
lex [] = []
lex str@(s:ss) | isDigit s = (Num . read . takeWhile isDigit $ str) : (lex $ dropWhile isDigit str)
               | isSpace s = lex $ dropWhile isSpace str
               | otherwise = Op s : lex ss

toOp :: Token -> StateT [Integer] IO ()

toOp t@(Num a) = do
  st <- get
  put (a:st)

toOp (Op o) = do
  st <- get
  case o of
    '+' -> put $ (st !! 0 + st !! 1) : (drop 2 st)
    'p' -> lift $ putStrLn $ show $ head st
    -- fill in the rest of ops e.g. -/*^ etc

rpnCalc :: String -> IO ()
rpnCalc str = evalStateT (sequenceA_ . map toOp . lex $ str) []

main = rpnCalc "1 2 + p"