Parser.hs

import Text.Parsec
import Text.Parsec.String

data Expr = Mul Expr Expr | Add Expr Expr | Lit Int deriving Show

expr :: Parser Expr
expr = try lit <|> try mul <|> try add <|> parenthesized
    where
    mul = char '*' >> Mul <$> rec <*> rec 
    add = char '+' >> Add <$> rec <*> rec
    lit = (Lit . read) <$> many1 digit
    rec = many space >> expr
    parenthesized = between (char '(') (char ')') rec