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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 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 example:

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

% direct call of p/1:

| ?- database::p(X).

X = 1 ; X = 2 yes

% solver - a simple meta-interpreter for pure Prolog:

| ?- database::solve(p(X)).

X = 1 ; X = 2 yes

| ?- database::proof_tree(p(X), Tree).

X = 1 Tree = p(1):- (q(1, a):- (s(1):-true), (t(1, a):-true)), (r(a):-true) ; X = 2 Tree = p(2):- (q(2, b):- (s(2):-true), (t(2, b):-true)), (r(b):-true) yes

% tracer - a simple meta-interpreter for tracing goal proofs using pure Prolog:

| ?- database::trace(p(X)). 1 call: p(_G180) 2 call: q(_G180, _G316) 3 call: s(_G180) 3 exit: s(1) 3 call: t(1, _G316) 3 exit: t(1, a) 2 exit: q(1, a) 2 call: r(a) 2 exit: r(a) 1 exit: p(1)

X = 1 ; 1 redo: p(1) 2 redo: r(a) 2 fail: r(a) 2 redo: q(1, a) 3 redo: t(1, a) 3 fail: t(1, _G316) 3 redo: s(1) 3 exit: s(2) 3 call: t(2, _G316) 3 exit: t(2, b) 2 exit: q(2, b) 2 call: r(b) 2 exit: r(b) 1 exit: p(2)

X = 2 ; 1 redo: p(2) 2 redo: r(b) 2 fail: r(b) 2 redo: q(2, b) 3 redo: t(2, b) 3 fail: t(2, _G316) 3 redo: s(2) 3 exit: s(3) 3 call: t(3, _G316) 3 fail: t(3, _G316) 3 redo: s(3) 3 fail: s(_G180) 2 fail: q(_G180, _G316) 1 fail: p(_G180)

no

% counting the number of resolution steps:

| ?- lists::steps(reverse([1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30],_), Steps).

Steps = 496 yes

% another example: expert system rules:

| ?- rules::prove(weather(Weather)).

Weather = raining yes

| ?- rules::prove(goto(Where)).

Where = cinema yes