Lazy evaluation is procrastination, doing work later rather than now. Why is this important? Well, it turns out if you procrastinate long enough, sometimes it turns out that the work never needed to be done after all!
Take, for example, a simple expression evaluation:
fn main()
{
2 + 3;
|| 2 + 3;
}
In a strict interpretation, the first expression will perform an arithmetic calculation. The second expression will define an arithmetic calculation but will wait before evaluating it.
This case is so simple that the compiler gives a warning and might choose to discard the unused constant expression. In more complicated cases, the lazy evaluated case will always perform better when not evaluated. This should be expected because unused lazy expressions do nothing, intentionally.
Iterators are lazy. They don't do anything until you collect or otherwise...
