sign :: Num n => Parser (n -> n)
sign = (negate <$ char '-') <|> (id <$ char '+') <|> return id
sign :: Num n => Parser (n -> n)
sign = (negate <$ char '-') <|> (id <$ char '+') <|> return id
sign = (negate <$ char '-') <|> (id <$ char '+') <|> return id