What's next after this? There are a lot of different projects ongoing for OCaml (typed effects, modular explicits/implicits, all of the RFCs, flambda and flambda 2, and I'm probably forgetting a lot of them). However, while following the progress of OCaml Multicore was easy thanks to the monthly posts, it's a bit harder to know where the focus will be next. Is there something like a "state of OCaml" that's planned, or a roadmap?

Congratulations on the merge, and thank you for all the work that went into it!

Few of those projects are still at the research stage (typed effects, modular implicits, unboxed types), with a handful of people working on them at most.

It is hard or even counter-productive to try to fit them on a roadmap.

The multicore project is(was?) a bit unusual from the point of view of OCaml development since it is a massive engineering effort with a focused team.

Nevertheless, I hope that we continue and extend the "OCaml compiler bimonthly" news to give more information about the ongoing work on the compiler.

Thank you for the explanation. I hope too you will continue the OCaml compiler bimonthly, it's always an interesting read, and it's nice to be up to date with what's happening.

For the not so OCaml savvy, can you recommend a blog post that explain, what programming constructs this adds

Maybe a before and after, but one that focus on expressiveness, what you were not able to say/express in OCaml before, that is now* possible

not just speed or performance

*s/not/now

Good question!

https://github.com/ocaml-multicore/effects-examples has links to tutorials and examples for how effects can be used.

There's also some slides from KC's talk on effect handlers https://kcsrk.info/slides/handlers_edinburgh.pdf and materials from the CUFP 17 tutorial: https://github.com/ocamllabs/ocaml-effects-tutorial

https://gopiandcode.uk/logs/log-bye-bye-monads-algebraic-eff... this is also a great introduction

What were the main technical challenges? why did it take so many years? did it take research breakthrough, or was it something else (organisational issue / technical debt...)?

It involved a fair amount of research and there were many technical challenges. Maintaining backwards compatibility in terms of language features and single-threaded performance puts constraints around what you can implement. Also from my personal experience, debugging segfaults in parallel programs that could be caused by GC bugs is a painstaking process.

We've written a couple of papers detailing the internals and the trade-offs involved: https://arxiv.org/abs/2004.11663 (for parallelism) and https://arxiv.org/abs/2104.00250 (for effects)

Added to that is the complexity of tracking a moving target. Multicore had to be rebased through 12 releases of OCaml, which in itself was a non-trivial amount of work.

Perhaps not the question you expected, but ..

> (for maybe the second to last time)

How come?

With the merging to trunk, Multicore doesn't really exist as a separate project. The last major milestone that is likely to have a multicore-dominant discussion thread is probably the 5.0 release.

As someone who has worked so long on the project, how do you feel? Relieved?

Hah. I only contributed to the project for about half it's life.

There's still a lot of work to be done before 5.0 is released, it's just it'll happen through the usual PR and review process rather than a separate one.

Maybe a bit of a stupid question, but, will the compiler itself evolve to use multiple cores?

Not a stupid question. I think adding parallelism to the compiler might be possible, though there are certainly parts that use a lot of mutable state that could prove tricky.

Whether it's beneficial or not is unclear though.

Can you recommend a good resource for getting started with OCaml ?

Real World OCaml (https://dev.realworldocaml.org/) is a great one.

Books, videos, and other good resources are listed here: https://ocaml.org/learn/

This course text is also great, I don't know if it's listed at ocaml.org/learn or not:

https://cs3110.github.io/textbook/

+1 for CS 3110 (OCaml Programming: Correct + Efficient + Beautiful), and the video series is similarly excellent: https://www.youtube.com/playlist?list=PLre5AT9JnKShBOPeuiD9b...