{-# LANGUAGE FlexibleContexts #-}

import Text.Parsec
import Text.Parsec.Expr
import Text.Parsec.Combinator
import Data.Functor
import Data.Function (on)

data Exp
  = Num Int
  | Add Exp
        Exp
  | Sub Exp
        Exp
  | Mul Exp
        Exp
  | Div Exp
        Exp

expr
  :: Stream s m Char
  => ParsecT s u m Exp
expr = buildExpressionParser table factor
  where
    table =
      [ [op "*" Mul AssocLeft, op "/" Div AssocLeft]
      , [op "+" Add AssocLeft, op "-" Sub AssocLeft]
      ]
    op s f = Infix (f <$ string s)
    factor = (between `on` char) '(' ')' expr <|> (Num . read <$> many1 digit)

eval
  :: Integral a
  => Exp -> a
eval (Num x) = fromIntegral x
eval (Add a b) = eval a + eval b
eval (Sub a b) = eval a - eval b
eval (Mul a b) = eval a * eval b
eval (Div a b) = eval a `div` eval b

solution
  :: Integral a
  => String -> a
solution = either (const (error "Did not parse")) eval . parse expr ""

main :: IO ()
main = print $ solution "(1+3)*7"