[yashrk]

Rc and Arc traits are implementations of the _runtime_ reference counters. Runtime reference counting is sometimes less efficient than tracing GC.

But Roc counts references in _compile_ time. So it's like _usual_ (not wrapped in Rc<>) values in Rust. But in Rust the value is deleted from the heap when the stack frame with the _only_ link to it («the owner») is deleted. And in Roc the value is deleted from the heap, when the _last_ link to it leaves the stack.

So we have the machine code _almost_ as efficient as a Rust-produced machine code, but the source code with a much simpler semantics.

[Yoric]

Do you have a written source for this? What you're describing is nice, but I can see so many basic cases in which it doesn't work, at least not without a borrow analysis much more sophisticated than Rust's, that I imagine I'm missing something.

Example: A linked list.

Nitpick: Rc and Arc are not traits.

[yashrk]

Two main papers about this topic AFAIK are: «Counting Immutable Beans» by Sebastian Ullrich and Leonardo de Moura (https://arxiv.org/pdf/1908.05647.pdf) and «Perceus: Garbage Free Reference Counting with Reuse» by Alex Reinking, Ningning Xie,Leonardo de Moura, Daan Leijen (https://dl.acm.org/doi/pdf/10.1145/3453483.3454032).

See also Anton Felix Lorenzen's master thesis: https://antonlorenzen.de/master_thesis_perceus_borrowing.pdf

Mr. Feldman also mentioned some «alias analysis» library «Morphic Solver» from Berkley University, but I cannot find anything about it.

BTW compile time reference counting was already discussed on HN, for example here: https://news.ycombinator.com/item?id=19567666

> Example: A linked list.

IIUC recursive data structures like linked list and trees are not the problem for the static reference counting. Mutation (and, therefore, link cycles) _is_ a problem, but Roc is a clean language with no explicit mutation. =)

> I can see so many basic cases in which it doesn't work

Sometimes runtime reference counting is still used. But, of course, in way more complex situations than just processing a linked list.

> Nitpick: Rc and Arc are not traits.

Of course, my fault. I haven't written on Rust for a long time (and never used this language in a serious project).

edit: link added

[Yoric]

Thanks!

[yashrk]

I found what is «Morphic», mentioned by Richard Feldman.

It's an another experimental programming language with the static reference counting. «Morphic uses a borrow-based reference counting scheme which is able to eliminate almost all reference count increments and decrements for a large class of programs»ⓒMorphic developer team.

Site: https://morphic-lang.org/

Paper: https://dl.acm.org/doi/10.1145/3591260

[Yoric]

Thanks, it makes sense now!

I think I've read a post by Niko Matskakis (Rust lead) who suggested doing something similar. That would be exciting to see!