Module Picos_sync.Event

First-class synchronous communication abstraction.

Events describe a thing that might happen in the future, or a concurrent offer or request that might be accepted or succeed, but is cancelable if some other event happens first.

See the Picos_select library for an example.

ℹ️ This module intentionally mimics the Event module provided by the OCaml POSIX threads library.

type !'a t

An event returning a value of type 'a.

type 'a event = 'a t

An alias for the Event.t type to match the Event module signature.

Composing events

val choose : 'a t list -> 'a t

choose events return an event that offers all of the given events and then commits to at most one of them.

val wrap : 'b t -> ('b -> 'a) -> 'a t

wrap event fn returns an event that acts as the given event and then applies the given function to the value in case the event is committed to.

val map : ('b -> 'a) -> 'b t -> 'a t

map fn event is equivalent to wrap event fn.

val guard : (unit -> 'a t) -> 'a t

guard thunk returns an event that, when synchronized, calls the thunk, and then behaves like the resulting event.

⚠️ Raising an exception from a guard thunk will result in raising that exception out of the sync. This may result in dropping the result of an event that committed just after the exception was raised. This means that you should treat an unexpected exception raised from sync as a fatal error.

Consuming events

val sync : 'a t -> 'a

sync event synchronizes on the given event.

Synchronizing on an event executes in three phases:

  1. In the first phase offers or requests are made to communicate.
  2. One of the offers or requests is committed to and all the other offers and requests are canceled.
  3. A final result is computed from the value produced by the event.

⚠️ sync event does not wait for the canceled concurrent requests to terminate. This means that you should arrange for guaranteed cleanup through other means such as the use of structured concurrency.

val select : 'a t list -> 'a

select events is equivalent to sync (choose events).

Primitive events

ℹ️ The Computation concept of Picos can be seen as a basic single-shot atomic event. This module builds on that concept to provide a composable API to concurrent services exposed through computations.

type 'a request = {
  1. request : 'r. (unit -> 'r) Picos.Computation.t -> ('a -> 'r) -> unit;
}

Represents a function that requests a concurrent service to update a computation.

⚠️ Raising an exception from a request function will result in raising that exception out of the sync. This may result in dropping the result of an event that committed just after the exception was raised. This means that you should treat an unexpected exception raised from sync as a fatal error. In addition, you should arrange for concurrent services to report unexpected errors independently of the computation being passed to the service.

val from_request : 'a request -> 'a t

from_request { request } creates an event from the request function.

val from_computation : 'a Picos.Computation.t -> 'a t

from_computation source creates an event that can be committed to once the given source computation has completed.

ℹ️ Committing to some other event does not cancel the source computation.