Interpreter for simple postfix expressions

postfix.hs

import Data.List
import Data.Char
import System.Environment

isOp :: String -> Bool
isOp x = x `isInfixOf` "+-*/"


isInt :: String -> Bool
isInt ('-':x:xs) = isInt (x:xs)
isInt x = all isDigit $ x


eval' :: [(String,String)] -> [Int] -> [Int]
eval' [("", _)] st = st
eval' [(x, xs)] st
 |isInt x = eval' (lex xs) $ ((read x) :: Int) : st
 |isOp x = case st of
            (b : a : ss) -> eval' (lex xs) ((doOp x a b) : ss)
            _ -> error "Stack too small!"
 |otherwise = error "Unknown token :("
 where doOp "+" a b = a + b
       doOp "-" a b = a - b
       doOp "*" a b = a * b
       doOp "/" a b = a `div` b
       doOp _ _ _ = error "Unknown operation!"
       
eval' [] st = st


eval :: String -> [Int]
eval str = eval' (lex str) []


main :: IO ()
main = do
  args <- getArgs
  case args of
    [exp] -> print . eval $ exp
    _ -> do
     pn <- getProgName
     putStrLn $ "Usage: " ++ pn ++ " <expr>"