Example: The Lwt Monad

By now, it's probably obvious that the Lwt promises library that we discussed is also a monad. The type 'a Lwt.t of promises has a return and bind operation of the right types to be a monad:

module type Lwt : Monad
  type 'a t
  val return : 'a -> 'a t
  val bind : 'a t -> ('a -> 'b t) -> 'b t

And Lwt.Infix.( >>= ) is a synonym for Lwt.bind, so the library does provide an infix bind operator.

Now we start to see some of the great power of the monad design pattern. The implementation of 'a t and return that we saw before involves creating references, but those references are completely hidden behind the monadic interface. Moreover, we know that bind involves registering callbacks, but that functionality (which as you might imagine involves maintaining collections of callbacks) is entirely encapsulated.

Metaphorically, as we discussed before, the box involved here is one that starts out empty but eventually will be filled with a value of type 'a. The "something more" in these computations is that values are being produced asynchronously, rather than immediately.