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Showing Selected Constraints (simple version)

Sometimes the programmer is not interested in every global constraint, only some selected ones. Such a filter can be easily implemented with a user-defined visualizer. Suppose that you are interested in the constraints all_different/1 and all_distinct/1 only:

%% spec_filter(+Constraint, +Actions):  Call fdbg_show for all constraints
%%   for which intresting_event(Constraint) succeeds.
%%
%%   Use this filter by giving the constraint_hook(spec_filter) option to
%%   fdbg_on.
spec_filter(Constraint, Actions) :-
        interesting_event(Constraint),
        fdbg_show(Constraint, Actions).

interesting_event(all_different(_)).
interesting_event(all_distinct(_)).

Here is a session using the visualizer. Note that the initialization part (domain/3 events), are filtered out, leaving only the all_different/1 constraints:

| ?- [library('clpfd/examples/suudoku')].
[...]
| ?- fdbg_on(constraint_hook(spec_filter)).
% The clp(fd) debugger is switched on
yes
% advice
| ?- suudoku([], 1, P).
all_different([1,<fdvar_1>,<fdvar_2>,8,<fdvar_3>,
               4,<fdvar_4>,<fdvar_5>,<fdvar_6>])
    fdvar_1 = 1..9 -> (2..3)\/(5..7)\/{9}
    fdvar_2 = 1..9 -> (2..3)\/(5..7)\/{9}
    fdvar_3 = 1..9 -> (2..3)\/(5..7)\/{9}
    fdvar_4 = 1..9 -> (2..3)\/(5..7)\/{9}
    fdvar_5 = 1..9 -> (2..3)\/(5..7)\/{9}
    fdvar_6 = 1..9 -> (2..3)\/(5..7)\/{9}

[...]

all_different([7,6,2,5,8,4,1,3,9])
    Constraint exited.

P = [...] ;
no
% advice
| ?- fdbg_off.
% The clp(fd) debugger is switched off

Note that the failure of spec_filter/2 doesn't cause any unwanted output.