Code shared from the Rust Playground

playground.rs

use core::iter::once;

fn main() {
    println!("{:?}", fmap(|x| x + 1, 1..100000));
}

fn fmap<A, B>(f: impl Fn(A) -> B, mut foo: impl Iterator<Item=A>) -> Vec<B> {
    match foo.next() {
        Some(x) => once(f(x)).chain(fmap(f, foo)).collect(),
        None => Vec::new()
    }
}