eioio VS domainslib

the actual project (Readme has some code samples): https://github.com/ocaml-multicore/eio

For 5.0+ you might want to look at https://github.com/ocaml-multicore/eio for how effects can make async much more pleasant

Great post! I would love to see this extended to OCaml 5 (with eio) and Haskell

Second, effects enable a new style of concurrency libraries like eio that forgoes the need to wrap every asynchronous computation in a monad.

> This is realllly unidiomatic in real world Haskell.

Whether idiomatic or not does not matter. It proves my point:

IO won't save you, and even very mundane effects are not part of the game…

Idris is the "better Haskell" sure, but the effect tracking is still part of the uncanny valley (still IO monad based).

Koka is a toy, and Frank mostly "only a paper" (even there is some code out there).

The "Frank concept" is to some degree implemented in the Unison language, though:

https://www.unison-lang.org/learn/fundamentals/abilities/

Having a notion of co-effects (or however you please to call them) is imho actually much more important than talking about effects (as effects are in fact neither values nor types—something that all the IO kludges get wrong).

I think the first practicable approach in the mainstream about this topic will be what gets researched and developed for Scala. The main take away is that you need to look at things form the co-effects side first and foremost!

In case anybody is interested in what happens in Scala land in this regard:

https://www.slideshare.net/slideshow/embed_code/key/aLE9M37d...

https://docs.scala-lang.org/scala3/reference/experimental/cc...

But also the development in OCaml seems interesting:

https://github.com/ocaml-multicore/eio#design-note-capabilit...

Look mom, "effects", but without the monad headache!

To enable access to all these features, an exciting new library called Eio is being developed. It uses a new paradigm of direct-style concurrent I/O programming, without the need for monads or async/await, thus avoiding the function colour problem.

For nested parallel computations (think Scientific Programming, where one would use OpenMP, Rust Rayon, etc), we have domainslib [1]. Eio, a direct-style, effect-based IO library is pretty competitive against Rust Tokio [2]. The performance will only get better as we get closer to the 5.0 release.

[1] https://github.com/ocaml-multicore/domainslib

[2] See the http server performance graphs at https://tarides.com/blog/2022-03-01-segfault-systems-joins-t...

1. Domains are the unit of parallelism. A domain is essentially an OS thread with a bunch of extra runtime book-keeping data. You can use Domain.spawn (https://github.com/ocaml-multicore/ocaml-multicore/blob/5.00...) to spawn off a new domain which will run the supplied function and terminate when it finishes. This is heavyweight though, domains are expected to be long-running.

2. Domainslib is the library developed alongside multicore to aid users in exploiting parallelism. It supports nested parallelism and is pretty highly optimised (https://github.com/ocaml-multicore/domainslib/pull/29 for some graphs/numbers). The domainslib repo has some good examples: https://github.com/ocaml-multicore/domainslib/tree/master/te...

3. We've not tested against other forms of parallelism. There isn't anything stopping you exploiting SIMD in addition to parallelism from domains.

4. No, we've not compared performance by OS.

5. No plans for the multicore team to look at accelerator integration at the moment.

[3] Domainslib -- Parallel Programming over Multicore OCaml, https://github.com/ocaml-multicore/domainslib