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set_event_handler(+EventId, ++PredSpec)
Set an event handler PredSpec for the event EventId.
- EventId
- Atom or Integer.
- PredSpec
- Term of the form Atom/Integer, or defers(Atom/Integer).
Description
Assigns the procedure specified by PredSpec as the event handler
for the event specified by EventId. The event name can be either
an arbitrary atom or a valid error number (as returned by
current_error/1).
An event handler which is used as an error handler can have 4
optional arguments:
- the 1st argument is the event number/identifier itself
- the 2nd argument is the culprit (a structure corresponding to
the call which caused it)
- the 3rd argument is the context module (or the lookup module
if the context module is unknown)
- the 4th argument is the lookup module for the call
The error handler is free to use less than 4 arguments.
Handlers for events raised by event/1 or posted to the system from
the outside usually have no arguments or just the event name.
Events can be raised by
- one of the builtins event/1, error/2 or error/3.
- posting an event from external code using ec_post_event().
- an interrupt whose handler has been specified as event/1.
The latter two have the effect of dynamically inserting an event/1
goal into the current execution at the next synchronous point,
which is usually just before the next predicate call.
If the handler is specified as defers(Name/Arity), then the event has
the defer-property. This means that event handling is automatically
deferred on entering the event's handler, thus preventing other events
from interrupting the handler. Such handlers must always explicitly invoke
events_nodefer/0 before exiting in order to reenable event handling.
Modes and Determinism
- set_event_handler(+, ++) is det
Modules
This predicate is sensitive to its module context (tool predicate, see @/2).
Exceptions
- (4) instantiation fault
- Either EventId or PredSpec is not instantiated.
- (5) type error
- EventId is neither atom nor integer.
- (5) type error
- PredSpec is not of the form Atom/Integer or defers(Atom/Integer).
- (6) out of range
- EventId is integer, but not a valid error number.
- (6) out of range
- PredSpec specifies an illegal handler arity.
Examples
Success:
?- event(hello).
warning: no handler for event in hello
yes.
?- set_event_handler(hello, writeln/1).
yes.
?- event(hello).
hello
yes.
See Also
current_error / 1, get_event_handler / 3, error / 2, error / 3, event / 1, reset_event_handler / 1, events_nodefer / 0