Function delay 

pub fn delay<I: Iterator, F: FnOnce() -> I>(new: F) -> Delayed<I, F> 

Wraps an iterator created by new on the first call to next().

For example, consider Haskell’s canonical zipWith implementation:

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

A naive translation to Rust would cause infinite recursion. We can emulate Haskell’s lazy evaluation by delaying the recursive calls:

use iter_utils::delay;

/// Returns an inefficient iterator over the Fibonacci numbers.
fn fibs() -> Box<dyn Iterator<Item = u32>> {
    Box::new(
        [0, 1]
            .into_iter()
            .chain(delay(fibs).zip(delay(fibs).skip(1)).map(|(x, y)| x + y)),
    )
}

for (got, want) in fibs().zip([0, 1, 1, 2, 3, 5, 8, 13]) {
    assert_eq!(got, want);
}