trait MonadOnce<T>: MonadMut<T> {
    fn bind<F, U: Default>(self, F) -> U
      where F: FnOnce(T) -> U;
}

trait MonadMut<T>: Monad<T> {
    fn bind<F, U: Default>(self, F) -> U
      where F: FnMut(T) -> U;
}

trait Monad<T> {
    fn bind<F, U: Default>(self, F) -> U
      where F: Fn(T) -> U;
}

mod impls {
    use super::{Monad, MonadMut, MonadOnce};

    impl<T, N: MonadOnce<T>> MonadMut<T> for N {
        fn bind<F, U: Default>(self, f: F) -> U
          where F: FnMut(T) -> U {
            MonadOnce::bind(self, f)
        }
    }

    impl<T, N: MonadMut<T>> Monad<T> for N {
        fn bind<F, U: Default>(self, f: F) -> U
          where F: Fn(T) -> U {
            MonadMut::bind(self, f)
        }
    }
}

impl<T> MonadOnce<T> for Option<T> {
    fn bind<F, U: Default>(self, f: F) -> U
      where F: FnOnce(T) -> U {
        if let Some(value) = self {
          f(value)
        } else {
            U::default()
        }
    }
}

fn main() {
    let foo = Some(2);
    println!("{:?}", Monad::bind(foo, |i| i + 1));

    let bar:Option<i32> = None;
    println!("{:?}", Monad::bind(bar, |i| i + 1));

    println!("{:?}", Monad::bind(bar, |i| Some(i + 1)));

}