rust/src/test/run-pass/monad.rs

impl monad<A> for ~[A] {
    fn bind<B>(f: fn(A) -> ~[B]) -> ~[B] {
        let mut r = ~[];
        for self.each {|elt| r += f(elt); }
        r
    }
}

impl monad<A> for option<A> {
    fn bind<B>(f: fn(A) -> option<B>) -> option<B> {
        alt self {
          some(a) { f(a) }
          none { none }
        }
    }
}

fn transform(x: option<int>) -> option<str> {
    x.bind({|n| some(n + 1)}).bind({|n| some(int::str(n))})
}

fn main() {
    assert transform(some(10)) == some("11");
    assert transform(none) == none;
    assert (~["hi"]).bind({|x| ~[x, x + "!"]}).bind({|x| ~[x, x + "?"]}) ==
        ~["hi", "hi?", "hi!", "hi!?"];
}