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Turns out it is not really new but only open sourced it now. From the FQA.

>Lore, formerly called Unreal Revision Control, is the built-in version control system for UEFN (Unreal Editor for Fortnite), where creators have been using it to version their islands. It is also seeing progressive adoption by internal Epic teams, and is being implemented as the backing store for UEFN’s cook pipeline, where it replaces traditional intermediary storage layers—eliminating redundant file transfers and significantly reducing the time between publishing changes and those changes being playable.

Surprised it is in Rust and not Epic C++ or Verse. I wonder why.

3 comments

I suspect the use of Rust rather than C++ might have something to do with the fact that Simon Peyton Jones and Lennart Augustsson (both of Haskell fame) both work at Epic and there would have been a strong internal push to do this in a language with some functional programming features. Rust rather than Verse because that would probably not be the right tool for the job (even if Simon works on it). Rust rather than Haskell probably because of performance -- DARCS never caught on partly for performance reasons.

I don't see how Rust is more functional programming oriented than C++

C++'s stdlib collection algorithms were basically all mutate-in-place instead of return-a-new-value until C++20 introduced ranges, and I still don't know anyone who actually uses those. By contrast, people actually use the `container.iter().map(...)` etc. in Rust.

C++ also lacks much of the fancy pattern matching features which are frequently associated with functional programming. Such features may not be fundamental to functional programming, but they would fall under "some functional programming features", in the sense that they're common in the languages of that ecosystem.

Rust has a load of FP-style iter/map/filter stuff that you can technically do in C++ but only if you don't mind bleeding eyes.

It also borrows a lot of features from FP languages like nice tagged unions (enums), Result, everything-is-an-expression, etc.

Guess I never used map+collect in Rust. It's an extra copy. Maybe iter on its own cause that's how you go through non-array collections, same in C++ except C++ is just crappy no matter how you use it.

I think the things that make Rust safer than C++ make it look more like a functional programming language than C++. The main point is that there are some influential people at Epic (SPJ has often spoken favourably about Rust) who would favour it over C++.

Pure by default, type classes (traits), no inheritance, type level lifetime computation. It's not only more functional it's much more type-safe and closer to Haskell in spirit

I like most of those decisions, but they aren't functional programming

It's a separate tool from Unreal Engine, so you don't need to constrain yourself to its conventions. You're also not going to use UObjects, its reflection layer, serialization, and all that other stuff for a tool that's strictly for version control. Plus you're going to tie yourself to all the issues and slowness of the engine. Epic made this mistake with the Epic Games Launcher, which is essentially an instance of the engine running, and a huge source of various issues.

As for Verse vs Rust, Verse is used in UEFN experiences, but it is _far_ from any kind of production state. It's also intrinsically tied to UEFN. It's not like you can download a standalone compiler or REPL for Verse.

Honestly it inspires rather more confidence in me to see a tool that has actually been used for a while and is just now being released for public use as well. Unless, of course, it's being released because they've decided to stop development, but I don't think that's the case here.