[kadoban]

Take your favorite immutable data structure. Fix it in your mind.

Now, call it mutable, but don't change a thing. Congratulations, it's now a mutable data structure that's exactly as fast as your favorite immutable one.

_But_ you now have extra opterations you can perform to optimize slow parts if you can. You have strictly more power than you started with, because you can modify data. You don't have to, but you can.

Just don't make any bad optimizations. If some attempt to make it better instead makes it worse, go back to the starting point and try something else.

[kephasp]

Do you have examples of this idea that were implemented and weren't insanely error-prone?

[kadoban]

The key is that you can start with any immutable data structure you like and change it as much as you want until it stops performing as desired. If that's errors (inability to reason about it correctly) or some other issue, doesn't really matter.

The point is that this procedure applies to anything. Immutable data structures are a strict subset of mutable ones. Every immutable data structure is open to moving any number of steps, in any direction, in the space of data structures. It's _purely_ extra freedom that immutable data structures don't have.

If you want specific ones, first things that would come to mind for me is to start with any immutable tree impl, and start adding small mutable ~caching things on top. Batch up updates a bit to try to avoid repeated deep traversals maybe. You can do this without adding much complexity at all, and it's very flexibly for whatever operations or access patterns you want to improve.

[kephasp]

Look for actual examples in the wild that weren't error prone and I'd be surprised you'll find any.

I get that you can imagine it. I'm saying it wouldn't work in practice.

[kadoban]

Searching wide and far for a data structure that's that niche in implementation is not really sensible, I'd probably have to implement one myself. If the sketch above doesn't satisfy you, that's about the end for me. Have a good one.

[kephasp]

You need to sketch it and possibly implement it yourself because actual examples don't exist. QED