________________________________________________________________________

This file is part of Logtalk <https://logtalk.org/>  
Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>  
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
________________________________________________________________________


% start by loading the loading the example:

| ?- logtalk_load(nondet(loader)).
...


% make a threaded call with a non-deterministic goal:

| ?- threaded_call(lists::member(X, [1,2,3])).

X = _G189 
yes

% retrieve through backtracking all solutions for the non-deterministic goal:

| ?- threaded_exit(lists::member(X, [1,2,3])).

X = 1 ;
X = 2 ;
X = 3 ;
no


% make a threaded call by committing to the first solution found:

| ?- threaded_once(lists::member(X, [1,2,3])).

X = _G189 
yes

% retrieve through backtracking the goal solution:

| ?- threaded_exit(lists::member(X, [1,2,3])).

X = 1 ;
no


% when two or more variant calls are made...

| ?- threaded_call(lists::member(X, [1,2,3])), threaded_call(lists::member(Y, [1,2,3])).

X = _G189 Y =_G190
yes

% ...the first threaded_exit/1 call will pick one of them:

| ?- threaded_exit(lists::member(X, [1,2,3])).

X = 1 ;
X = 2 ;
X = 3 ;
no

% ...and a second threaded_exit/1 call will pick the remaining one:

| ?- threaded_exit(lists::member(X, [1,2,3])).

X = 1 ;
X = 2 ;
X = 3 ;
no


% tags may be used to distinguish between threaded calls if needed:

| ?- threaded_call(lists::member(X, [1,2,3]), Tag).

Tag = 1
yes

| ?- threaded_call(lists::member(X, [1,2,3]), Tag).

Tag = 2
yes

| ?- threaded_exit(lists::member(X, [1,2,3]), 2).

X = 1 ;
X = 2 ;
X = 3 ;
no


% use a subsumed goal instead of a variant of the original goal:

| ?- threaded_call(lists::member(X, [1,2,3,2])).

X = _G189 
yes

| ?- threaded_exit(lists::member(2, [1,2,3,2])).

More ;
More ;
no
