[wg-parallel] About Lwt and Async

Jeremie Dimino jdimino at janestreet.com
Mon Apr 29 11:18:57 BST 2013

Hi everybody and welcome to the wg-parallel mailing list,

The aim of this group is to discuss the future of monadic cooperative
concurrent programing in OCaml.  There are currently two well-known
libraries for that purpose, lwt and async. Lwt is historically the
first one, async was developed at Jane Street and has been
open-sourced recently.

These libraries are quite 'invasive' since they require to write the
code in a cooperative way.  It is not possible to use both of them in
the same program and even if a third-party library can sometime be
made to work with both, it is in general complex and not very

So while they are both important having two such 'core' and
incompatible libraries is not a very good thing for the OCaml
community.  Switching brutally from one to the other is not realistic
since it would require a huge amount of time and efforts.  The idea
would be to make them evolve so they are close enough and, if
possible, eventually have only one.

One strategy developped by big projects using both lwt and async such
as Mirage/Xen is to interact at the RPC level.  This can be done on
top of lwt and async without modifying them and there are people
working in this direction.

They are currently two big semantic divergences in the kernel of both
libraries: execution order and error handling. Plus they are a number
of other more or less important differences.  I try to describe them
in the rest of this email.

Semantic of bind

The general idea in lwt is to run the most it can immediatly and
yielding is always explicit, while in async the precedence is given to
knowing exactly when the code is going to be executed.

So for [return x >>= f]:

- lwt evaluates [f x] immediatly
- async returns a non-determined deferred and will evaluate [f x]
during the next cycle

This semantic of the bind is very important for lwt users, but async
is already planning to add a second monad with lwt's behavior, so it
shouldn't be an issue.

Lwt and async also differs regarding Lwt.wait/Ivar.fill but there
seems to be an agreement that the async's behavior is better and it is
trivial to change in lwt so that shouldn't be an issue.

Error handling

The ['a Lwt.t] type 'inline' the error monad [Value of 'a | Error of
exn] while in async all errors are explicit.  So the async equivalent
of ['a Lwt.t] would be [('a, exn) Result.t Deferred.t].

As a result, while most functions in lwt return a [something Lwt.t],
in async they return a variant or a [something Or_error.t Deferred.t]
when the result may be an error status.

This may be tedious to handle.

Local storage

Lwt allows to define global variables with different values in
different threads.  This was introduced so logs can display 'thread
identifiers' but is now also used for other things.  Async internally
has such a mechanism, it could be adapted and exported.


The kernel of lwt can be used without a scheduler, which is important
for javascript for instance.  The scheduler is part of the lwt.unix
package and can be run multiple times in the lifetime of a program.

This is not true for async but it is something that can be considered.


The kernel of lwt depends only on the standard library and has no C.
As a result it is very portable and can be used without problem in
javascript or any other exotic environment.

The kernel of async (Async_core) currently depends on Core which
depends on Unix and contains C bindings.  It is planned to split Core
to get a small kernel for Core and Async.

Lwt is also more portable, for example it runs on Windows.  Windows
support could be added to Async.

There are questions about whether this should be done like in
unix/lwt.unix, i.e. emulate unix's behavior on Windows, or binds the
Win32 API directly and write the dispatching code in OCaml.

General experience

Async is part of the core suite and use core's convention.  Code using
it starts with:

    open Core.Std
    open Async.Std

which brings all the core and async world into the environment.

The core style is generally considered better and async has a huge
number of utilities well integrated together, but switching code to it
may require a lot of work.

On the other hand lwt is more independent so it is easier to integrate
it into an existing project.

In conclusion there is a lot to think about but no blocker.


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