from https://en.wikipedia.org/wiki/Functional_programming
I totally agree that neither iterators nor 2/3s of closures are "functional programming". That's fine, though, because I really enjoy the sane way that it all was done.
If you check out
https://doc.rust-lang.org/std/iter/trait.Iterator.html
and search for `FnMut`, you see that most of the iterator methods are side-effectful. That is cool and fun and very useful, but it isn't functional programming.
Edit. The term you are probably looking for (guessing) is "higher-order programming":
That said, you're right that "higher order" is a better description of these features, but my counter would be that most people perceive higher order programming as an aspect of functional programming, bringing it full circle again :)
I think it's fair to say that iterator methods are typically used as combinators to build computations out of pure functions to avoid having to manage mutable state.
And then, what specifically does "functional programming" mean to you that makes implementing an interface a feature from functional paradigm?
But this all misses the point of my comment. If you knew it would invoke the unnecessary discussion to call the iterator "functional feature" (as the quoted comment signifies), you should have rephrased the title of the chapter in the first place, rendering the disclaimer unnecessary.
