- Reference manual
If goal_expansion/2 wraps a goal as in the example below the system still reaches fixed-point as it prevents re-expanding the expanded term while recursing. It does re-enable expansion on the arguments of the expanded goal as illustrated in t2/1 in the example.59After discussion with Peter Ludemann and Paulo Moura on the forum.
:- meta_predicate run(0). may_not_fail(test(_)). may_not_fail(run(_)). goal_expansion(G, (G *-> true ; error(goal_failed(G),_))) :- may_not_fail(G). t1(X) :- test(X). t2(X) :- run(run(X)).
Is expanded into
t1(X) :- ( test(X) *-> true ; error(goal_failed(test(X)), _) ). t2(X) :- ( run((run(X)*->true;error(goal_failed(run(X)), _))) *-> true ; error(goal_failed(run(run(X))), _) ).
Note that goal expansion should not bind any variables in the clause. Doing so may impact the semantics of the clause if the variable is also used elsewhere. In the general case this is not verified. It is verified for \+/1 and ;/2, resulting in an exception.