Create an iterator from a closure representing a recurrence relation.

recurrence_relation_iter.rs

use std::mem;

struct RecRel<T,F> {
    val: T,
    succ: F,
}

impl<T,F> RecRel<T,F> {
    fn new(init: T, succ: F) -> Self where F: Fn(&T) -> T {
        RecRel {
            val: init,
            succ: succ,
        }
    }
}

impl<T,F> Iterator for RecRel<T,F> where F: Fn(&T) -> T {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        let next = (self.succ)(&self.val);
        Some(mem::replace(&mut self.val, next))
    }
}

fn main() {
    let fib = RecRel::new((0,1), |&(a,b)| (b,a+b));

    for (i,(n,_)) in fib.enumerate().take(10) {
        println!("F[{}] = {}", i, n);
    }
}