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

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