:- use_module(library(aggregate)).(can be autoloaded)
foreach(Generator, Goal) :- findall(Goal, Generator, Goals), maplist(call, Goals).
The actual implementation uses findall/3 on a template created from the variables shared between Generator and Goal. Subsequently, it uses every instance of this template to instantiate Goal, call Goal and undo only the instantiation of the template and not other instantiations created by running Goal. Here is an example:
?- foreach(between(1,4,X), dif(X,Y)), Y = 5. Y = 5. ?- foreach(between(1,4,X), dif(X,Y)), Y = 3. false.
The predicate foreach/2
is mostly used if Goal performs backtrackable destructive
assignment on terms. Attributed variables (underlying constraints) are
an example. Another example of a backtrackable data structure is in
If we care only about the side effects (I/O, dynamic database, etc.) or
the truth value of Goal, forall/2
is a faster and simpler alternative. If Goal instantiates its
arguments it is will often fail as the argument cannot be instantiated
to multiple values. It is possible to incrementally grow an
?- foreach(between(1,4,X), member(X, L)). L = [1,2,3,4|_].
Note that SWI-Prolog up to version 8.3.4 created copies of Goal using copy_term/2 for each iteration.