13.2 Calling Prolog from JavaScript

The Prolog class provides several methods for calling Prolog from JavaScript.

Boolean Prolog.call(Goal)

Processes a Prolog goal represented as a String and returns true or false. This simple calling pattern is intended for trivial goals such as setting a Prolog flag. For example, the call below limits the Prolog stacks to 10Mb.

Prolog.call("set_prolog_flag(stack_limit, 10 000 000)");

Query Prolog.query(Goal)

Query Prolog.query(Goal, Input)

Query Prolog.query(Goal, Input, Options)

Create a Prolog query from a String, optionally binding Prolog variables embedded in Goal from properties of the Object Input. The returned object is an instance of class Query. This instance can be used as a JavaScript iterator. The value returned in each iteration is an Object with properties for each variable in Goal that is not in Input and does not start with an underscore. For example, we can iterate over the members of a list like below. Further details on class Query are provided in section 13.2.1. The translation of data between Prolog and JavaScript is described in section 13.2.3.

for(let r of Prolog.query("member(Elem,List)",
                          {List: ["aap", "noot", "mies"]}))
{ console.log(r.Elem);
}

This interface is also practical for calling (fast) Prolog predicates to compute a single answer from an input using the Query.once() method. Assuming a Prolog predicate fib/2 that computes the nth Fibonacci number, we can call this using the code below. Note that if the fib/2 fails or raises an exception the object returned by Query.once() does not contain the Out key and thus our function returns undefined.

function fib(in, out)
{ return Prolog.query("fib(In,Out)", {In:in}).once().Out;
}

The .query() method is indented for fast queries that do not require the yield mechanism, i.e., the execution should not require asynchronous operations and the browser is not responsive during the execution. Use Prolog.forEach() for asynchronous queries.

The optional Options parameter defines the following options

engine: Boolean

If true, run the query in a temporary engine. Note that JavaScript can only interact with the innermost query of an engine. By using a new engine we can interact with multiple queries, using them as coroutines. Prolog.Engine() for details.

Promise Prolog.forEach(Goal, [Input], [OnAnswer], [Options])

This method executes Goal asynchronously. This implies that Goal may execute asynchronous operations and the browser remains responsive while executing Goal. Goal and Input are processed as with Prolog.query(). If OnAnswer is provided, this Function is called with a Object that holds the bindings for the output arguments of Goal for each answer. Options supports the following options

engine: Boolean

If true, create an engine that will be destroyed when the query completes.

heartbeat: Integer

Sets the heartbeat rate. This is the number of Prolog inferences executed before yielding. The default is 10,000. Lower values improve interactive behaviour but lower Prolog performance.

All default parameters may be omitted. A JavaScript type check is used to discover whether the Input or OnAnswer parameter is omitted. Use an empty Input to specify Options without inputs, e.g. forEach(goal, {}, {engine:true})

The returned Promise is resolved when the query completes. The value passed to the .then() method of the Promise is the number of answers if OnAnswer is provided or an Array of answers if OnAnswer is omitted. If Goal raises an exception the Promise is rejected.

Multiple calls to Prolog can be activated on the same engine at any time. Prolog processes such queries in LIFO (Last In, First Out) mode. If queries need to be processed sequentially use JavaScript await or the Promise.finally() method to wait for completion. Multiple Prolog.forEach() queries that run in separate engines act as cooperative threads, i.e., all make progress. For example, a goal can be started to run as cooperative thread using:

  setTimeout(async () => {
    await Prolog.forEach(goal, input, onanswer,
                         {engine:true});
  });

13.2.1 The JavaScript class Query

The method Prolog.query() returns an instance of the JavaScript class Query that may be used to explore the solutions of the query. The Query class implements the JavaScript iterator protocol.

Object Query.next()

Implements the iterator protocol. This returns an object holding the keys done and value. If exception handling is enabled it returns an object {done:true, error:true, message:String}.

Object Query.once()

Close the query after the first answer. Returns the .value of the object returned by .next() on success and the complete object on failure or error. In addition, on a logical result (no error), a field success is added with a boolean value. Thus, the return value may contain these keys:

{Bindings}

Query succeeded. Objects holds a key for each output variable. In addition the success key is set to true.

{success:false}

Query failed. Note that the binding keys all start with an uppercase letter and this is thus not ambiguous.

{error:true, message:String}

Query raised an exception.

Object Query.close()

Closes the query. This can be used inside the iterator to stop further enumeration.

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.

13.2.3 Translating data between JavaScript and Prolog

JavaScript and Prolog are both dynamically typed languages. The WASM module defines a faithful translation between JavaScript data and Prolog data that aims at completeness as well as keeping the data representation clean in the common cases. We describe the translation in two descriptions because round tripping does not always result in the original object.

13.2.3.1 Translating JavaScript data to Prolog

This section describes how data from JavaScript is translated into Prolog. The interface is primarily designed for passing JavaScript data as typically used to a natural Prolog representation. In addition a number of classes are provided to create Prolog specific data structures such as strings (as opposed to atoms), variables, compound terms, etc.

Number

Translate to a Prolog integer or floating point number.

BigInt

Translate to a Prolog integer.

String

Translate to a Prolog atom. Use new Prolog.String(text) to create a Prolog string. See below.

Boolean

Translate to one of the Prolog atoms true or false.

undefined

Translate the Prolog atom undefined.

null

Translate the Prolog atom null.

Array

Translate to a Prolog list.

Objects holding the key $:Type

Such objects are converted depending on the value for this key. The interface defines classes to simplify creating such objects.

s

Represent a Prolog string. The key v holds the text. May be created using new Prolog.string(text). May be created using new Prolog.String(text).

r

Represent a Prolog rational number. The keys n and d represent the numerator and denominator. For example, to represent 1r3, use {$:"r", n:1, d:3}. May be created using new Prolog.Rational(n, d), where n and d can be JavaScript numbers or big integers.

t

Represent a Prolog compound term. The object should hold exactly one key whose value is an array that holds the argument values. For example a term point(1,2) is constructed using {$:"t", point:[1,2]}. May be created using new Prolog.Compound(functor, args)

v

Represent a variable. If the key v is present this identifies the variable. Two variables processed in the same translation with the same identifier represent the same Prolog variable. If the v key is omitted the variable will be unique. May be created using new Prolog.Var(id).

l

Represent a Prolog list. As a JavaScript Array we only need this typed object to create a partial list. The v key contains the “normal” elements and the key tail contains the tail of the list. May be created using new Prolog.List(array, tail).

Object of class Object

Plain JavaScript objects are translated into a Prolog dict. Note that JavaScript object keys are always strings and (thus) all dict keys are atoms. This, {1:"one"} is translated into _{'1': one}.

ArrayBuffer

Instances of ArrayBuffer are translated into a Prolog string that consists of characters in the range 0 ... 255.

Objects of a one class not being Object

Instances of non-plain JavaScript objects are translated into a Prolog blob. Such objects are written as <js_Class(id)>. The Prolog interface allows for passing the objects back and calling methods on them. See section 13.3.

13.2.3.2 Translating Prolog data to JavaScript

Most of the translation from Prolog data to JavaScript is the reverse of the translation described in section 13.2.3.1. In some cases however reverse translation is ambiguous. For example, both 42 and 42n (a JavaScript BigInt) translate to a simple Prolog integer. The other way around, as JavaScript Number is a float, both Prolog 42 and 42.0 translate to 42 in JavaScript.

Variable

Translate to a JavaScript Prolog.Variable instance where the identifier is a unique number of each unique variable.

Integer

Translate to a JavaScript Number when possible or BigInt otherwise. Currently JavaScript Number can represent integers up to 2^53 precisely.

Rational

Translate to a JavaScript Prolog.Rational instance.

Float

Translate to a JavaScript Number.

Atom

Translate to a JavaScript String.

String

Translate to a JavaScript Prolog.String instance.

List

When a proper list create a JavaScript Array, otherwise create a JavaScript Prolog.List instance.

Compound term

Create a JavaScript Prolog.Compound instance.

Dict

Create a plain JavaScript Object with the same keys. If the dict has a non-var tag, add a $tag property.