Re: [Caml-list] Question about Lwt/Async

[Ashish Agarwal <agarwal1975@gmail.com>]

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> Also, what happens to general utility functions that aren't rewritten for
Async/Lwt -- as far as I can tell, being in non-monadic code, they will
always starve other threads, since they cannot yield to another Async/Lwt
thread.

There is Lwt_preemptive.detach and Async's In_thread.run to get around this.


> It seems that Lwt is more popular in the community outside JaneStreet
than Async (if only by looking at its reverse dependencies on opam.ocaml.org).
There has been posts about this, for instance
http://rgrinberg.com/blog/2014/12/11/abandoning-async/ .

I'd be wary of drawing conclusions from one blog post and even from opam. I
think the answer is: both are used a lot. Also depends on what you mean by
"a user". It's not too useful to count Jane Street's packages and one
barely used package on opam both as 1. A lot of code is not on opam.


> Is there an existing compatibility library functorized over the
intersection of Async and Lwt? That would make being compatible with both
much easier.

Most people provide this internally for each of their projects, e.g. Cohttp's
IO signature <https://github.com/mirage/ocaml-cohttp/blob/master/lib/s.mli>.
However, we have quite a few projects that needed this abstraction, so
duplicating this code in each repo seemed wrong. Thus we developed future
<https://github.com/solvuu/future>, which was recently released in opam.



On Mon, Mar 7, 2016 at 9:06 AM, Yotam Barnoy <yotambarnoy@gmail.com> wrote:

> Is there an existing compatibility library functorized over the
> intersection of Async and Lwt? That would make being compatible with both
> much easier.
>
> On Mon, Mar 7, 2016 at 4:08 AM, Simon Cruanes <simon.cruanes.2007@m4x.org>
> wrote:
>
>> Hi,
>>
>> It seems that Lwt is more popular in the community
>> outside JaneStreet than Async (if only by looking at its reverse
>> dependencies on opam.ocaml.org). There has been posts about this, for
>> instance http://rgrinberg.com/blog/2014/12/11/abandoning-async/ .
>> However, if you're writing a library, it is good taste (if possible) to
>> parametrize you code over an "IO" monad that will be easy to instantiate
>> with either Async or Lwt (or the trivial blocking monad where 'a t = 'a
>> and (>>=) x f = f x) along with the required IO primitives.
>>
>> Regarding general utility functions, if they do not perform IO or depend
>> on (blocking) IO they can be used directly with Async/Lwt (unless they
>> really take a very long time to complete).
>>
>> Le Mon, 07 Mar 2016, Malcolm Matalka a écrit :
>> > Yotam Barnoy <yotambarnoy@gmail.com> writes:
>> > > Hi all
>> > >
>> > > I'm thinking about my next project in OCaml, and I'm wondering how
>> many
>> > > users of OCaml currently use Lwt or Async regularly.
>> > >
>> > > One of the advantages of OCaml over Haskell (which I'm not crazy
>> about) is
>> > > the fact that you don't have to constantly be stuck inside a monad.
>> > > However, once you want to use these user-level threading libraries,
>> you're
>> > > essentially tied to a monad. It also means that the usage of any other
>> > > monad from Lwt/Async code is out -- OCaml doesn't have the monad
>> > > transformer infrastructure to layer monads easily as far as I can
>> tell (am
>> > > I wrong?). I mean, even in Haskell using Monad Transformers is a pain
>> (IMO).
>> > >
>> > > Also, what happens to general utility functions that aren't rewritten
>> for
>> > > Async/Lwt -- as far as I can tell, being in non-monadic code, they
>> will
>> > > always starve other threads, since they cannot yield to another
>> Async/Lwt
>> > > thread. Is this perception correct? If so, this seems to imply that
>> you
>> > > either write your code to cooperate within these frameworks and
>> suffer the
>> > > monad, or don't, and make it near-impossible for Lwt/Async users to
>> make
>> > > use of your code.
>> > >
>> > > I would like to get an idea of the usage level of these libraries, as
>> well
>> > > as the burden of writing compatible code, any difficulties etc. Also,
>> I'd
>> > > like to get a sense of the domains that benefit from these libraries.
>> Some
>> > > domains (such as gaming) traditionally involve a continuous main
>> loop, and
>> > > would thus only suffer from the additional overhead of queuing in
>> these
>> > > libraries.
>> > >
>> > > -Yotam
>> >
>> > I mostly use Async.  However, I think most usage of Lwt or Async
>> > requires doing as little as possible in these frameworks and using them
>> > to orchestrate other functions.  For example, I usually try to separate
>> > parsing of a network protocol from the reading and writing of the bytes.
>> >
>> > --
>> > Caml-list mailing list.  Subscription management and archives:
>> > https://sympa.inria.fr/sympa/arc/caml-list
>> > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>> > Bug reports: http://caml.inria.fr/bin/caml-bugs
>>
>>
>> --
>> Simon Cruanes
>>
>> http://weusepgp.info/
>> key 49AA62B6, fingerprint 949F EB87 8F06 59C6 D7D3  7D8D 4AC0 1D08 49AA
>> 62B6
>>
>
>

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