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Class type Uq_engines.engine

class type ['a] engine = object .. end
This class type defines the interface an engine must support. The class parameter 't is the type of the result values (when the engine goes to state `Done).

Requirements for engines

method state : 'a engine_state
Returns the state of the engine
method abort : unit -> unit
Forces that the engine aborts operation. If the state is already `Done , `Aborted, or `Error, this method must do nothing (you cannot abort an already finished engine).
method request_notification : (unit -> bool) -> unit
Requests notification about state changes.

After the notification has been requested, the passed function must be called whenever state changes its value (or might change its value; it is allowed to call the notification function more frequently than necessary). The function returns true if there is still interest in notification, and false if notification must be disabled; the function must not be called any longer in this case.

There can be any number of parallel active notifications. It is allowed that a notification callback function requests further notifications.

If the callback raises an exception, this exception is propagated to the caller of

method event_system : Unixqueue.event_system
Returns the event system the engine is attached to
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