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A delayed computation that can produce a deferred.
Nothing happens with a lazy deferred unless one forces it. Forcing a lazy deferred starts the computation, which will eventually cause the deferred to become determined. As usual with laziness, multiply forcing a lazy deferred is no different than forcing it a single time.
Exceptions (both synchronous and asynchronous) raised by a delayed computation are returned by force (wait, peek, etc.), or will be raised to the monitor in effect when force_exn (wait_exn, peek_exn, etc.) was called.
The type is not exposed nor defined as 'a Deferred.t Lazy.t or 'a Or_error.t
Deferred.t Lazy.t, because there is a difference in power with these types. There is no way to hook an asynchronous computation to a lazy value in OCaml to be triggered when the lazy gets computed. This functionality is indeed offered by this module (see wait). Plus, dealing with exception raised by the closures provided is slightly easier when done consistently through this API.
There is no val of_lazy : 'a Deferred.t Lazy.t -> 'a t because of the difference in power.