First math parser

sad_parser.hs

import Data.Char
import System.IO
import System.Exit
import Control.Applicative

symbol = flip elem ['+', '-', '*', '/']
getOp c = case c of
  "+" -> (+)
  "-" -> (-)
  "*" -> (*)
  "/" -> div

split [] cur open close = [reverse cur]
split (x:xs) cur open close
  | x == '(' = split xs (if open == 0 then cur else nxt) (open + 1) close
  | x == ')' = split xs (if open == close + 1 then cur else nxt) open (close + 1)
  | open == close && symbol x = reverse cur : [x] : split xs "" 0 0
  | otherwise = split xs nxt open close
  where nxt = x : cur

non :: Num b => a -> b -> b
non a b = b

strip str = go str 0 non
  where
    go :: [String] -> Int -> (Int -> Int -> Int) -> [String]
    go [] res _ = [show res]
    go (x:xs) res op
      | x == "*" || x == "/" = go xs res (getOp x)
      | x == "+" || x == "-" = show res : x : go xs 0 non
      | otherwise = go xs (op res (sol x)) non

sol :: String -> Int
sol str = if head lis == str then (read str :: Int) else go (strip lis) 0 non
  where
    lis = (split str "" 0 0)
    go [] res _ = res
    go (x:xs) res op
      | x == "+" || x == "-" = go xs res (getOp x)
      | otherwise = go xs (op res (sol x)) non

main = do
  putStr "> "
  hFlush stdout
  s <- getLine
  if length s /= 0
    then print $ sol s
    else exitWith ExitSuccess
  main