I believe xi-rope counts as a persistent data structure. We have plug-ins in separate processes now, for isolation, but might explore having them in-process but in separate threads. The underlying data structure should support that just fine. Is there something I'm missing?

A rope data structure is very almost persistent. So perhaps I am splitting hairs! It could of course be made fully persistent if mutating operations are avoided. This would allow sharing them between threads.

> This would allow sharing them between threads.

This is an area where Rust ends up feeling different than many languages: xi's rope uses https://doc.rust-lang.org/stable/std/rc/struct.Rc.html#metho... like this: https://github.com/google/xi-editor/blob/422948d62688dcc2ee0... , which gives you both options. This code uses Rc, not Arc, so it's not currently sharable between threads, but Arc has the same API.

Rust cares about sharing more than immutability.

Nope, it's Arc for exactly that reason, though we're not currently using it across threads right now. The fact that it can support immutable semantics safely is one of the really nice features of Rust.

Thanks willtim, for mentioning Ewig/Immer :)

As Ralph and others have pointed out, their rope is fundamentally a persistent data structure.

I think that one of the fundamental differences between how Rust and C++ deal with value and reference semantics make the two approaches look more different than they are. The C++ API is allowed to look a bit more "immutable", where one can always use the same functional looking API even though sometimes the performance of an operation is gonna depend on the category of the reference you are operating on (l-value, r-value). Rust forces the user to be explicit about everything all the time, making it very explicit when intend to "branch" or "snapshot" the persistent data structure and where do we wanna allow "in place" updates.

I am still undecided about which approach I like most, and I do think that there merits to each approach. But maybe just because I never really wrote anything serious in Rust. The C++ approach has the problem that it becomes unsafe once you introduce `std::move` (but if the C++ type system was not a crazy evolutionary monster one could imagine a similar approach with moves being inserted automatically and safely by the compiler.)