{-# OPTIONS_GHC -fglasgow-exts -Wall #-}

module Rpn (evaluate) where

import Char

isStrDigit :: String -> Bool
isStrDigit = all isDigit

-- Check that a string matches regex /^-?\d+$/.
isSNum :: String -> Bool
isSNum []        = False
isSNum "-"       = False
isSNum ('-':xs)  = isStrDigit xs
isSNum xs        = isStrDigit xs

calc :: Int -> String -> Int -> Int
calc x "+" y  = x+y
calc x "-" y  = x-y
calc x "*" y  = x*y
calc x "/" y  = x`div`y
calc _ tok _  = error $ "Invalid token:" ++ show tok

evalStack :: [Int] -> String -> [Int]
evalStack xs y
  | isSNum y        = (read y):xs
  | (a:b:cs) <- xs  = (calc b y a):cs
  | otherwise       = error "Stack underflow"

evaluate :: String -> Int
evaluate expr
  | [e] <- el       = e
  | otherwise       = error $ "Invalid stack:" ++ show el
  where
  el = foldl evalStack [] $ words expr