# \[ANN\] Lwt.6.0.0~alpha (direct-style) **URL:** \ **Category:** Ecosystem\ **Tags:** monad, lwt, effects\ **Created:** [July 15, 2025, 9:50am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972 "2025-07-15T09:50:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 15, 2025, 9:50am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/1 "2025-07-15T09:50:32Z")
It is a great pleasure to announce the release of the first alpha release of Lwt 6. This major version bump brings two major changes to Lwt: - Using Lwt in direct-style! (Big thanks to @c-cube !!) - Using multiple Lwt schedulers running in separate domains! ### Direct-style This contribution from @c-cube is available in alpha00. It comes in the form of an `lwt_direct` package which provide an `Lwt_direct` module which provide two core functions: ```ocaml val run : (unit -> 'a) -> 'a Lwt.t val await : 'a Lwt.t -> 'a ``` and allows you to write code such as ```ocaml run (fun () -> let continue = ref true in while !continue do match await @@ Lwt_io.read_line ic with | line -> await @@ Lwt_io.write_line oc line | exception End_of_file -> continue := false done) ``` There are a few more functions. All of which is documented in [lwt\_direct.mli](https://github.com/ocsigen/lwt/blob/6.0.0-alpha00/src/direct/lwt_direct.mli). ### Multi-scheduler This addition is not available in alpha00 but should be added to alpha01 soon. It allows to call `Lwt_main.run` in different domains and benefit from actual parallelism. (Sneak peek in [this pull request](https://github.com/ocsigen/lwt/pull/1058)) ### Installation `lwt.6.0.0~alpha00` and `lwt_direct.6.0.0~alpha00` will soon be released on opam ([PR on opam-repo](https://github.com/ocaml/opam-repository/pull/28175). I’ll publish some more alphas as the work progresses, and announce the releases on this thread. You can also pin the packages to the lwt-6 branch to get everything a little bit earlier: ```bash opam pin lwt https://github.com/ocsigen/lwt.git#lwt-6 opam pin lwt_direct https://github.com/ocsigen/lwt.git#lwt-6 ``` ### Feedback Don’t hesitate to chime in on here with any feedback you may have. Ideas, comments, requests, suggestions, etc. ---
**Author:** ![R\_Huxton](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/r_huxton/32/4216_2.png) [@R\_Huxton](https://discuss.ocaml.org/u/R_Huxton)\ **Post date:** [July 15, 2025, 1:29pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/2 "2025-07-15T13:29:25Z")
> [@raphael-proust](#): > > ```auto > run (fun () -> > let continue = ref true in > while !continue do > match await @@ Lwt_io.read_line ic with > | line -> await @@ Lwt_io.write_line oc line > | exception End_of_file -> continue := false > done) > > ``` I’m guessing this should be `while continue do`, but it looks like an attractive option. ---
**Author:** ![c-cube](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/c-cube/32/1727_2.png) [@c-cube](https://discuss.ocaml.org/u/c-cube)\ **Post date:** [July 15, 2025, 1:36pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/3 "2025-07-15T13:36:48Z")
No, it’s `!continue`, to read the reference 🙂 . There are multiple ways to write this loop, but here it emphasizes the (new) possibility of writing direct style code that runs on Lwt’s scheduler and can wait on any `Lwt.t` promise. ---
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 15, 2025, 1:40pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/4 "2025-07-15T13:40:24Z")
As @c-cube mentions, it’s to read the reference. See [OCaml library : Stdlib](https://ocaml.org/manual/5.3/api/Stdlib.html#VAL(!)) for official documentation on `!` especially and [OCaml - The core language](https://ocaml.org/manual/5.3/coreexamples.html#s:imperative-features) for general info on imperative programming in OCaml. ---
**Author:** ![R\_Huxton](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/r_huxton/32/4216_2.png) [@R\_Huxton](https://discuss.ocaml.org/u/R_Huxton)\ **Post date:** [July 15, 2025, 4:14pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/5 "2025-07-15T16:14:46Z")
> [@c-cube](#): > > No, it’s `!continue`, to read the reference D’oh! The perils of working in different languages all the time. I still can’t read it as anything other than “while not” even now - the habit is too deeply ingrained. It’s switching between semicolon-terminated and not that mostly gets me for the first hour or two. Thanks ---
**Author:** ![vrotaru](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/vrotaru/32/39_2.png) [@vrotaru](https://discuss.ocaml.org/u/vrotaru)\ **Post date:** [July 15, 2025, 5:52pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/6 "2025-07-15T17:52:29Z")
I’m reminded of someone humorous explanations which may help with the correct interpretation. > How do you get a value out of a reference? > You bang(!) it. ---
**Author:** ![yawaramin](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yawaramin/32/3384_2.png) [@yawaramin](https://discuss.ocaml.org/u/yawaramin)\ **Post date:** [July 15, 2025, 8:50pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/7 "2025-07-15T20:50:22Z")
Question: why in a separate `Lwt_direct` module? Why not in the main `Lwt` module itself? Is it because of the name `run`? Could we use `Lwt.defer` and `Lwt.now` instead? ---
**Author:** ![c-cube](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/c-cube/32/1727_2.png) [@c-cube](https://discuss.ocaml.org/u/c-cube)\ **Post date:** [July 16, 2025, 2:35am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/8 "2025-07-16T02:35:45Z")
In the current state of things, `lwt_direct` requires OCaml 5 while `lwt` doesn’t. I think otherwise it could make sense for `Lwt_direct` to live in the `lwt` library, just maybe not in the `Lwt` module itself — after all it’s not about promises as much as the moral equivalent of microtasks. It’s an add-on to `lwt` offering a different way of thinking, while keeping full compatibility with the ecosystem. A separate module seems reasonable to me 🙂 I’m not sure why you would pick names like `defer` and `now`. `defer` sounds like a resource handling function, and `now` like a clock function? ---
**Author:** ![yawaramin](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yawaramin/32/3384_2.png) [@yawaramin](https://discuss.ocaml.org/u/yawaramin)\ **Post date:** [July 16, 2025, 2:47am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/9 "2025-07-16T02:47:24Z")
Oh OK, so the question is should Lwt 6 keep supporting OCaml 4? `defer` is used in ZIO-Direct, which is an equivalent feature in Scala’s ZIO effect system: [Introduction to ZIO Direct Style | ZIO](https://zio.dev/zio-direct/#introduction) `now` is just a contrast to ‘defer’, as in ‘get the value later’ vs ‘get the value now’. ‘Defer’ terminology is also used by Jane Street Async–their promise type is `'a Deferred.t`. Maybe `async` and `await` would actually be better choices, but there is already `Lwt.async` so it might be confusing. ---
**Author:** ![c-cube](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/c-cube/32/1727_2.png) [@c-cube](https://discuss.ocaml.org/u/c-cube)\ **Post date:** [July 16, 2025, 2:51am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/10 "2025-07-16T02:51:28Z")
> [@yawaramin](#): > > Oh OK, so the question is should Lwt 6 keep supporting OCaml 4? I think it’s a good question, since @raphael-proust has in the works a multicore PR (one Lwt event loop per domain). Even then I think a separate module would be good, inside the `lwt` library. Thanks for explaining `defer` and `now`. I think `await` is non-negotiable, it’s almost universal at this point. `run` is a bit generic and I could imagine there being better names! ---
**Author:** ![yawaramin](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yawaramin/32/3384_2.png) [@yawaramin](https://discuss.ocaml.org/u/yawaramin)\ **Post date:** [July 16, 2025, 2:54am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/11 "2025-07-16T02:54:56Z")
Yeah, I can imagine there being confusion about `Lwt_main.run` vs `Lwt_direct.run`. ---
**Author:** ![yawaramin](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yawaramin/32/3384_2.png) [@yawaramin](https://discuss.ocaml.org/u/yawaramin)\ **Post date:** [July 16, 2025, 3:02am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/12 "2025-07-16T03:02:59Z")
One more question: does `try await promise with ex -> ...` do the equivalent of `Lwt.catch` ie handle promise rejection? ---
**Author:** ![smondet](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/smondet/32/111_2.png) [@smondet](https://discuss.ocaml.org/u/smondet)\ **Post date:** [July 16, 2025, 3:08am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/13 "2025-07-16T03:08:57Z")
> [@yawaramin](#): > > Why not in the main `Lwt` module itself? May be worth mentioning, the `Lwt` module is available and very useful in the js-of-ocaml world where effects-compilation is even younger (even kinda experimental?). ---
**Author:** ![c-cube](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/c-cube/32/1727_2.png) [@c-cube](https://discuss.ocaml.org/u/c-cube)\ **Post date:** [July 16, 2025, 3:24am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/14 "2025-07-16T03:24:02Z")
Yes, `Lwt_direct.await promise` will raise the exception `e` if `promise` failed with `e`. It all works normally, `try … with`, `match … with exception …`, awaiting inside `List.map`, etc. Effects are awesome! ---
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 16, 2025, 6:23am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/15 "2025-07-16T06:23:19Z")
Yes, as mentioned by @c-cube the hope is to keep lwt compatible with ocaml4. It’ll likely require some more work on the multi-domain-multi-scheduler feature but I think it should be doable. As for names, I think a separate module is nice, I can see `Lwt_direct.run` being renamed `Lwt_direct.direct`, or maybe the introduction of `Lwt_direct.Syntax` module which you are meant to open and which provides `direct` and `await` and `yield` so you write `let open Lwt_direct.Syntax in direct (… bunch of code with awaits …)`. Suggestions definitelly welcome. ---
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 16, 2025, 6:24am UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/16 "2025-07-16T06:24:20Z")
Oh kind of off-topic but is there a summary somewhere about how it’s done? Also, does it use generators at all? ---
**Author:** ![cvine](https://avatars.discourse-cdn.com/v4/letter/c/90db22/32.png) [@cvine](https://discuss.ocaml.org/u/cvine)\ **Post date:** [July 16, 2025, 12:13pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/17 "2025-07-16T12:13:42Z")
> [@raphael-proust](#): > > ### Feedback > > Don’t hesitate to chime in on here with any feedback you may have. Ideas, comments, requests, suggestions, etc. Is the release of Lwt-6 a suitable opportunity to drop the `Lwt_process` module, as I don’t think it gives `Lwt` a good look? That is because that module can no longer be safely used, as `Lwt` implicitly starts `Thread.t` threads when encountering blocking calls, and `Lwt_unix.set_default_async_method` is no longer able to change this in recent versions of `Lwt`. The problem is that `Lwt_process` calls up `Unix.fork` (via `Lwt_unix.fork`) and also `Unix.exec*`, none of which is thread safe. About `Unix.fork` the OCaml documentation says “`[Unix.fork]` fails if the OCaml process is multi-core (any domain has been spawned). In addition, if any thread from the `Thread` module has been spawned, then the child process might be in a corrupted state.” About the `Unix.exec*` functions, those can allocate memory with malloc and so are not async-signal-safe and so not thread/domain safe ([Unix.create\_process\_env might not be multi-thread safe · Issue #12395 · ocaml/ocaml · GitHub](https://github.com/ocaml/ocaml/issues/12395)). There are also some things in `Lwt_unix` which are unsafe and could be removed for similar reasons, such as `Lwt_unix.fork` and `Lwt_unix.system`. ---
**Author:** ![lindig](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/lindig/32/532_2.png) [@lindig](https://discuss.ocaml.org/u/lindig)\ **Post date:** [July 16, 2025, 12:20pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/18 "2025-07-16T12:20:04Z")
> [@raphael-proust](#): > > Don’t hesitate to chime in on here with any feedback you may have. Ideas, comments, requests, suggestions, etc. To build an intuition, I would welcome an explanation what `run` and `await` do; also, it seems this can’t be implemented with OCaml 4 - but why is that? ---
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 16, 2025, 12:33pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/19 "2025-07-16T12:33:11Z")
I’ll check what uses of `Lwt_process` and `Lwt_unix.fork` there are in the ~~wild~~ opam-repository and consider that. Thanks for the suggestion! ---
**Author:** ![raphael-proust](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/raphael-proust/32/415_2.png) [@raphael-proust](https://discuss.ocaml.org/u/raphael-proust)\ **Post date:** [July 16, 2025, 12:42pm UTC](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972/20 "2025-07-16T12:42:34Z")
The gist of it is: `run : (unit -> 'a) -> 'a Lwt.t` creates a promise, sets up an effect handler to support `await` during the execution of its argument, executes its argument, resolves the promise with the result that is returned. `await : 'a Lwt.t -> 'a` takes a promise as argument. Because of the way the Lwt promises work, that promise’s evaluation is either a resolved promise or a promise waiting for the scheduler to be awoken (when IO is ready or at the next tick). In the former case `await` simply returns the value. In the latter case `await` performs an effect. Upon receiving this effect, the handler installed by `run` attaches the `continue` callback to the waiting promise (via a simple `Lwt.on_any`). More specifically, the handler installs a callback to the promise, the callback puts the `continue` inside a queue, the queue is iterated upon when the scheduler starts a new tick. There is a bit more to it, like exceptions. But that’s the gist of it. This is not possible in OCaml 4 because you can’t punch through the scheduler, do some side-effects, and return to wherever you happened to be at. [Next page](https://discuss.ocaml.org/t/ann-lwt-6-0-0-alpha-direct-style/16972.md?page=2)