[wg-parallel] About Lwt and Async

[David Scott]

Hi,

In the XenServer part of Citrix we're starting to use Core for some 
projects (and no doubt we'll incorporate Async into those soon) and 
we're also using Lwt, mainly via mirage. We haven't invested so much in 
either that it would be impossible to switch, although it will take a 
while: shipping code will often be maintained for around 5 years.

I had a quick look at our development opam repo[1] and, at the package 
level at least, our stuff breaks down roughly into

   * 25% C bindings for bits of Unix; traditional style; potentially 
blocking operations.
   * 25% protocol implementations / format converters. These are a mix 
of traditional style, with some lwt creeping in (often kept separate 
using ocamlfind subpackages)
   * 50% apps / services / tools. I'm expecting both Async and Lwt to 
start creeping in here.

Our apps usually talk to each other via RPC so we can live with a 
mixture of Async apps and Lwt apps but it would obviously be simpler if 
we were able to standardise (eventually) on one API.

The simple Lwt shim on top of Async Jeremie described would potentially 
be useful for us: in some of our less performance-critical apps we could 
clearly live with a bit of extra boxing. We're also not using very much 
of the Lwt library so even a very basic integration would still be useful.

Our performance critical apps will use mirage and its ecosystem of 
libraries, all based on Lwt. If it were possible to port the kernel of 
Async to mirage then we could use the Async engine everywhere, and
slowly port everything over to it.

Cheers,
Dave

[1] https://github.com/xen-org/opam-repo-dev