[steveklabnik]

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 :)

[frankmcsherry]

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"?

[steveklabnik]

The "oh no OOP" chapter is later :)

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