A Rust `unfold` function, to build an iterator by repeatedly applying a closure.

unfold.rs

fn main() {
    //let i = (0i32..).scan((0,1), |s,_| { *s = (s.1, s.0 + s.1); Some(s.0) });
    let i = unfold((0, 1), |(a, b)| Some(((b, a+b), b)));

    let v: Vec<i32> = i.take(20).collect();
    println!("{:?}", v);
}

struct Unfolder<F, T>(F, Option<T>);

impl<T, U, F> Iterator for Unfolder<F, T>
    where F: FnMut(T) -> Option<(T, U)>
{
    type Item = U;
    fn next(&mut self) -> Option<U> {
        self.1.take()
            .map_or(None, &mut self.0)
            .map_or(None, |(next_v, item)| {
                self.1 = Some(next_v);
                Some(item)
            })
    }
}

fn unfold<T, U, F>(t: T, f: F) -> impl Iterator<Item=U>
    where F: FnMut(T) -> Option<(T, U)>
{
    Unfolder(f, Some(t))
}