Yeah that is one way of doing it, but I'm not sure how practically useful it actually is; many functional languages do allow for mutation and/or side effects too. Often in a controlled way, of course, but then again, so does Rust. That said, I wouldn't argue that Rust is a functional language exactly, mostly that I find essentialist FP definitions to be lacking, like most essentialist definitions.

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 :)

Most people perceive structs and fields as an aspect of object-oriented programming, but you probably wouldn't write about them under the heading of "Object-oriented programming".

Maybe change the title to "Functional Language Idioms"?

The "oh no OOP" chapter is later :)

That's a good suggestion, I'll think about it, thanks :)

`FnMut` just means that the iterator methods don't artificially restrict the user-supplied closures. `map` itself doesn't become impure just because you can map `|x| { counter += x; x+1 }` over your list if you really want to.

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.