I feel APIs like that are a bit misleading in Rust in many cases, though, since they're also mutating the original. In Rust, if you want to perform a functiona

I feel APIs like that are a bit misleading in Rust in many cases, though, since they're also mutating the original. In Rust, if you want to perform a functional update like that you should take the type by move, but almost everyone just uses mutating APIs because mutation is explicit in the signature anyway. I think that's one of the biggest differences in practice between Rust and other MLs (with which it otherwise has a lot in common): in most MLs, mutation is treated as a last resort, while in Rust it's extremely common because it (generally) doesn't have nonlocal effects.