## 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)
  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 13.2.2.

      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.

Promise Prolog.forEach(Goal)
Promise Prolog.forEach(Goal, OnAnswer)
Promise Prolog.forEach(Goal, Input)
Promise Prolog.forEach(Goal, Input, OnAnswer)
  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 `.query()`. For each answer, OnAnswer is a `Function` that is called with a `Object` that holds the bindings for the output arguments of Goal.

  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 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.

### 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 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.

#### 13.2.2.1 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.

#### 13.2.2.2 Translating Prolog data to JavaScript

Most of the translation from Prolog data to JavaScript is the reverse of the translation described in section 13.2.2.1. 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.