The prolog_listen/2
interface allows for tracking new facts that are added to monotonic
tables. For example, we can print new possible connections from the
above program using this code:
:- prolog_listen(connected/2, connection_change).
connection_change(new_answer, _:connected(From, To)) :-
format('~p and ~p are now connected~n', [From, To]).
Currently, failure of the hook are ignored. If the hook
throws an exception this is propagated. The hook is executed outside the
current tabling context.188The
final behavior may be different in both aspects.
After loading the connected/2
program and the above declarations we can observe the interaction below.
Note that query 1 establishes the dependencies and fills the tables
using normal tabling. In the current implementation, possibly discovered
connections do not trigger the hook.189This
is likely to change in the future.. Adding a single link/2
fact links both locations to itself and to each other in both
directions. Adding a second fact extends the network.
1 ?- connected(_,_).
false.
2 ?- assert(link('Amsterdam', 'Haarlem')).
'Amsterdam' and 'Haarlem' are now connected
'Amsterdam' and 'Amsterdam' are now connected
'Haarlem' and 'Amsterdam' are now connected
'Haarlem' and 'Haarlem' are now connected
true.
3 ?- assert(link('Leiden', 'Haarlem')).
'Leiden' and 'Haarlem' are now connected
'Haarlem' and 'Leiden' are now connected
'Amsterdam' and 'Leiden' are now connected
'Leiden' and 'Amsterdam' are now connected
'Haarlem' and 'Leiden' are now connected
'Leiden' and 'Haarlem' are now connected
'Leiden' and 'Amsterdam' are now connected
'Leiden' and 'Leiden' are now connected
'Amsterdam' and 'Leiden' are now connected
true.
