enum Either<L, R> {
  Left(L),
  Right(R)
}

impl <L, R> Either<L, R> {
  fn cata<T, F: FnOnce(&L) -> T, G: FnOnce(&R) -> T>(&self, l: F, r: G) -> T {
    match *self {
      Either::Left (ref val) => l(val),
      Either::Right(ref val) => r(val)
    }
  }
}

fn main() {
  let val: Either<&str, &str> = Either::Right("lol");
  println!("{}", val.cata(|l| "", |r| r));
}