13.2.2 Using engines

The WASM version of SWI-Prolog supports engines. The initial engine is called main. Additional engines can be used to enumerate answers of multiple open queries as well as for implementing coroutines. Combined with JavaScript async functions, engines can provide cooperative multi-threading. For example, to enumerate the answers of two queries we may use the following code

  const e1 = new Prolog.Engine({auto_close:true});
  const e2 = new Prolog.Engine({auto_close:true});

  const q1 = e1.query(Query1);  // see Prolog.query()
  const q2 = e2.query(Query2);

  try
  { for(;;)
    { const n1 = q1.next();
      const n2 = q2.next();
      if ( n1.done && n2.done )
        break;
      // Handle answers
    }
  } finally
  { q1.close(); // also closes e1
    q2.close();
  }
  ...

Engines can also be used to create a cooperative thread. The example below creates a Prolog task that prints the numbers 1..20 to the console, one number every second.

  ...
  setTimeout(async () => {
    const e = new Prolog.Engine({auto_close:true});
    await e.forEach("between(1,20,X),sleep(1)",
                    (a) => console.out(a.X));
  });

Engine Prolog.Engine([name], [options])

Create a new engine. The name argument names the engine. When omitted, engines are named engineN, where N is defined by a counter. The options argument provides additional configuration. Both arguments are optional.

Boolean auto_close

When true (default false, closing the last query associated with the engine also closes the engine.

undefined close()

Terminate the engine. This may be called safely multiple times on the same instance.

Boolean Engine.call(Goal)

Object query(...args)

Object forEach(goal, ...args)

Any with_frame(function, persist)

Same as the corresponding methods on class Prolog, using the specified engine for running the Prolog goals.

Any with(function)

Run function using the specified Engine instance.