SWI-Prolog -- first

Availability::- use_module(library(thread)).(can be autoloaded)

[semidet]first_solution(-X, :Goals, +Options)

Try alternative solvers concurrently, returning the first answer. In a typical scenario, solving any of the goals in Goals is satisfactory for the application to continue. As soon as one of the tried alternatives is successful, all the others are killed and first_solution/3 succeeds.

For example, if it is unclear whether it is better to search a graph breadth-first or depth-first we can use:

search_graph(Grap, Path) :-
         first_solution(Path, [ breadth_first(Graph, Path),
                                depth_first(Graph, Path)
                              ],
                        []).

Options include thread stack-sizes passed to thread_create, as well as the options on_fail and on_error that specify what to do if a solver fails or triggers an error. By default execution of all solvers is terminated and the result is returned. Sometimes one may wish to continue. One such scenario is if one of the solvers may run out of resources or one of the solvers is known to be incomplete.

on_fail(Action): If stop (default), terminate all threads and stop with the failure. If continue, keep waiting.
on_error(Action): As above, re-throwing the error if an error appears.

bug: first_solution/3 cannot deal with non-determinism. There is no obvious way to fit non-determinism into it. If multiple solutions are needed wrap the solvers in findall/3.