[torginus]

I think functional just means that there's no undefined behavior. SSA shows that you can compile a procedural language to mostly functional constructs, and functional languages likewise have introduced things like mutation, where you can write to variables as if they were mutable and the compiler figures it out for you.

On the issue of copy-to-modify, if you can prove the old copy will never be used after you modify it, it's perfectly safe to implement it as an in place modification.

How do you prove/enforce this? With tracking ownership+lifetimes, like Rust does. In fact I'd argue that this makes Rust a functional language (no UD), and Rust isn't slow.

[hunterpayne]

Nit...Functional means there are no side effects. Side effects are defined, they just aren't easy to reason about.

I do want to point out the expected result of almost every single program an app dev has ever been paid to do is entirely defined as a collection of side effects. For example all I/O is a side effect. I know people have crammed I/O into frameworks and defined them as pure, but that's mostly handwaving.