?- X in 0..1, Y in 2..4, Z #= max(X,Y).
clpz:(X in 0..1), clpz:(Z#=max(X,Y)), clpz:(Z in 2..4), clpz:(Z#>=Y), clpz:(Y in 2..4). % unexpected.
?- X in 0..1, Y in 2..4, Z #= max(Y,X).
Y = Z, clpz:(X in 0..1), clpz:(Y in 2..4).
These non-terminals take a grammar rule body and additional arguments
as arguments. These arguments are appended to the first argument.
A key motivation for the introduction of these non-terminals is found
in the discussion and sample code provided by @bakaq in:
https://github.com/mthom/scryer-prolog/discussions/2260
In this way, portable higher-order DCG programming is possible while
keeping the logical grammar rule expansion implementation dependent.
Example:
?- phrase(phrase('.', a, []), Cs).
Cs = "a".
This allows subsequently invoked constraints to take the entire
filtering results into account, instead of being invoked when the
obtained information is not yet entirely used.
The SICStus-style attributed variables mechanism of Scryer Prolog
automatically prevents very subtle interaction problems that can arise
in systems that do not give all variables that are involved in a
unification an opportunity to schedule their propagators.
An example of such a subtle interaction is:
?- tuples_in([[A,C,B]], [[3,1,3],[4,2,4]]),
global_cardinality([A,B,D], [3-1,4-2]),
A = 4.
A = 4 causes pgcc_check/1 and pgcc/2 to be queued in the fast and slow
queue, respectively. In the fast queue, there is also rel_tuple/2,
which is worked off after gcc_check/1 and simultaneously instantiates
both C and B (to 2 and 4, respectively). Instantiation of C schedules
do_queue//0 from verify_attributes/3. Note that C does not participate
in the global_cardinality/2 constraint.
Critically, B also gets an opportunity to schedule its propagators in
this case, so another gcc_check/1 is run before gcc_global/2!
This allows subsequently invoked constraints to take the entire
filtering results into account, instead of being invoked when the
obtained information is not yet entirely used.
It speeds up programs such as the one in:
https://github.com/triska/clpz/issues/26
Markus Triska