LWT

Lwt is a lightweight concurrency library for the OCaml programming language, focused on cooperative threading and asynchronous I/O. It provides promises and a monadic interface that lets developers compose asynchronous operations without blocking the executing thread. A value of type Lwt.t represents a computation that will yield a result in the future, enabling non-blocking workflows. Programs combine such computations using operators like bind (>>=) and map (>|=), or with the let%lwt syntax extension for clearer sequential style.

The core idea of Lwt is an event-driven scheduler that runs the promises cooperatively. There is no preemptive multi-threading by default; instead, tasks yield control to the scheduler, which then continues other work. The main event loop is driven by Lwt_main.run, which executes the main Lwt thread and related asynchronous tasks until they complete. Lwt provides a rich library of combinators for parallelism and error handling, including Lwt.async to start a side-effecting computation, Lwt.yield to yield control, and facilities to cancel or fail promises. Additional utilities include Lwt.task for creating a promise with an explicit resolver, and helpers like Lwt.both and Lwt.all to coordinate multiple promises.

Lwt integrates with non-blocking I/O through modules such as Lwt_unix and Lwt_io, enabling asynchronous file and network operations. It is widely used for building network servers, clients, and other I/O-bound applications in OCaml. While Lwt enables scalable concurrency within a single process, it relies on cooperative scheduling, so CPU-bound work should be delegated to separate threads or processes to avoid blocking the event loop. Compared to other OCaml concurrency libraries, Lwt emphasizes composable asynchronous programming with a strong emphasis on declarative error handling.

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