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Module Equeue

module Equeue: sig .. end
Equeue implements generic event queues. An event system consists of an event queue, a list of event handlers, and an event source. The queue has FIFO semantics, i.e. the first event is processed next, and new events are appended to its end. When an event is delivered to an event handler, all handlers are tried until a handler accepts the event (or the event is dropped if no such handler can be found). See below how a handler can indicate whether to accept or reject an event.

When the queue is empty, the event source is called once. The source can add events to the queue, in order to continue event processing. If the queue remains empty, the execution of the event system stops.


The module can be used in multi-threaded program provided no event system is shared by several threads, or if so, access to functions is serialized.

type 'a t 
This is the type of an event system with events of type 'a
exception Reject
May be raised by event handlers to reject events
exception Terminate
May be raised by event handlers to accept events while terminating themselves
exception Out_of_handlers
Raised by run when the event source adds new events to the queue but there are no event handlers to process them
val create : ?string_of_event:('a -> string) -> ('a t -> unit) -> 'a t
Creates a new event system that has an event source, but is otherwise empty. The argument of type 'a t -> unit is the event source. The source can call add_event to put new events into the queue.

string_of_event : Optionally, one can pass a printer for events. This has only an effect for debugging output.
val add_event : 'a t -> 'a -> unit
Puts an event into the event queue of the system.
val add_handler : 'a t -> ('a t -> 'a -> unit) -> unit
Adds a handler to the list of handlers of the system.

An event handler is called with the event system and the event as arguments. The handler can return in various ways:

  • Return normally: This means that the event is accepted by the handler. No other handler will be asked to process the event.
  • Raise Reject: The event is rejected by the handler. The other handlers are asked to process the event.
  • Raise Terminate: The event is accepted, but the handler is terminated, i.e. will never be called again.
  • Raise another exception: The event is deferred, and will be processed again in the future, but after the already queued events. Furthermore, the exception falls through to the caller of run.
The handler can add new events and new event handlers. The latter will be activated when the next event is processed.
val run : 'a t -> unit
Running a system means that, unless the queue is empty, the events at the time of the run invocation and all later added events are gone through. Each event is presented to the handlers until one handler accepts the event. Events rejected by all handlers are dropped silently. If there is no pending event the default event source is called once. If there are still no events the system stops and returns. If there are events to process but no handlers which can do them all events are silently dropped, and the default event source is called once.

The exception Out_of_handlers is raised if there are events but no handlers after the event source has been called. This is considered as a programming error, and would cause infinite looping if not detected.

Note that there is an implicit order among the handlers which is simply the order the handlers have been added to the system. This means that you can set a fallback handler which catches any unprocessed event by adding it last.

Note that the events are processed in the order they happen. There is no mechanism to assign priorities to events.

Handlers are allowed to raise arbitrary exceptions. Exceptions other than Reject and Terminate are not caught, so the caller has to do this if appropriate. It is possible to restart an event system by just calling run again.

val is_running : 'a t -> bool
Returns whether the event loop is active
module Debug: sig .. end
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