Tau-Prolog : enhancing sample

Tau-Prolog is an implementation of Prolog in pure Javascript.

Here is a basic example :

// Create a Prolog session
let program = `
    parent(john, mary).
    parent(susan, mary).

    parent(mary, alice).
    parent(mary, bob).
    parent(paul, bob).

    grandparent(X, Y) :- parent(X, Z), parent(Z, Y).

    sibling(X, Y) :- parent(Z, X), parent(Z, Y), X \\= Y.

    ancestor(X, Y) :- parent(X, Y).
    ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).
`;

function showAnswer(answer, context) {
    if (pl.type.is_substitution(answer)) {
        for (let key in answer.links) {
            document.write(answer.links[key].id + "<br>"); // Extract only the name
        }
    } else {
        console.log("no answer");
    }
}

class PrologSession {
    constructor() {
        this.session = pl.create(1000);
        this.consultationDone = false;
        this.consultationResolve = null;

        // Load Prolog program
        this.session.consult(program, {
            success: () => {
                this.consultationDone = true;
                if (this.consultationResolve) {
                    this.consultationResolve(); // ✅ Resolves any pending queries
                    this.consultationResolve = null; // Prevents multiple calls
                }
            },
            error: (err) => {
                console.error("Prolog consultation error:", err);
            }
        });
    }

    waitForConsultation() {
        return new Promise((resolve) => {
            if (this.consultationDone) {
                resolve(); // If already done, resolve immediately
            } else {
                this.consultationResolve = resolve; // Store resolve function
            }
        });
    }

    query(queryString, context) {
        document.write(context + "<br />");
        this.waitForConsultation().then(() => {
            this.session.query(queryString, {
                success: () => {
                    this.session.answers((a) => showAnswer(a, context));
                },
                error: (err) => {
                    console.error("Query error:", err);
                }
            });
        });
    }
}

// Example usage
const prolog = new PrologSession();
prolog.query("ancestor(X, bob).", "Ancestors of Bob are: ");

Demo