Future - Abstraction over Stdlib, Lwt, and Async.

OCaml has two concurrency libraries in wide use: Lwt and Async. When writing your own library, the question is which of the two your API should be developed against. For example, if you provide a non-blocking function, should it return a Lwt.t or Async's Deferred.t? Ideally, you could provide both because that allows users of both Lwt and Async to use your new library.

In addition, sometimes you want to provide a blocking implementation based on OCaml's Stdlib. Blocking APIs are easier for beginners to understand and sometimes provide faster code (if your program doesn't have much concurrency, the overhead of Lwt and Async is wasted time).

This library provides a signature Future.S that abstracts over Stdlib, Lwt, and Async, and 3 implementations of it: Future_std, Future_lwt, and Future_async. The goal is to have Async.Std immediately satisfy this interface, i.e. to make Future_async = Async.Std. Thus, by functorizing your code over this interface, you can assume you are programming with Async (albeit with many fewer functions), and get Lwt and Stdlib versions of your code for free.

We do not succeed in making Future.S an exact subset of module type of Async.Std. Sometimes a Lwt or Stdlib implementation of some Async construct is not possible or difficult. Such deviations are kept to a minimum and documented. Usually, if a feature cannot be supported uniformly for Stdlib, Lwt, and Async, then it is not included here. There is no goal to be comprehensive. Async is a large library and we are not attempting to provide compatible Stdlib and Lwt versions for all of it.