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Explainer for Typed Objects

Outline

The explainer proceeds as follows:

Overview

Typed Objects add a new type of objects to JavaScript: objects with pre-defined storage for member fields with equally pre-defined types. This proposal focuses on mutable Struct types, but the concept is applicable to Value types, too.

Characteristics of Struct types

Struct Types have these characteristics:

  • Fixed layout: a Struct's layout is fixed during construction, i.e. it is sealed during its entire lifetime.
  • Indexed typed member fields: a Struct has as own members an indexed list of typed fields, as given in its definition.
  • Possible field types: typed fields can hold values as described in the section on Value Type definitions, including references to other Struct type instances.
  • Named aliases as prototype accessors: typed fields can optionally be given a—String or Symbol—name, in which case an accessor is installed on the prototype.
  • Support for recursive types: Struct types can be forward-declared and filled in later, enabling support for—directly or indirectly—recursive types.
  • Inheritance: Struct types can extend other Struct types (but not other JS classes/constructor functions). Additional typed fields are appended to the end of the parent type's indexed list.
  • Prototypes are exotic objects which forbid the definition of properties that shadow typed fields, or have numeric index names.
  • Immutable prototype chain—[[SetPrototypeOf]] throws when applied to any members of the Struct prototype chain.

See individual sections for more details on these characteristics.

Types

The central part of the Typed Objects specification are Value Types and Struct Types. Instances of Value Types are immutable and don't have internal structure or object identity: they are primitive values similar to numeric values like 10.5 or 42, or strings like "foo". Instances of Struct Types are objects like { answer: 42 }, except they have a fixed layout in memory, and each field is defined such that it can hold an instance of a Value Type (and only that, instead of arbitrary values).

Value Types

The following common Value Types are provided by this spec:

uint8  int8          any
uint16 int16         string
uint32 int32 float32 object
uint64 int64 float64 ref(T)

They're exposed as Function objects with the internal methods [[Read]] and [[Write]], which convert between Value and internal representations for the type. [[Write]] additionally performs a type-specific type check. Calling the Value Type object as a function executes the steps of the internal [[Write]] method followed by those of the internal [[Read]] method. I.e., it applies the steps for checking and converting from Value into the type's internal representation, and then back to Value.

Host environments can provide additional kinds of Value Types, with their own implementations of [[Read]] and [[Write]].

For all Value Types provided by this specification, [[Read]] converts from the internal representation to a value and returns it. The behavior of [[Write]] for all types provided by this specification is described below.

For the numeric types and the string type, the implementations of [[Write]] apply coercions: they ensure that the given value is of the right type by coercing it. For numeric types, the coercion is identical to that applied when writing to an element in a Typed Array. For string, it's identical to that applied when coercing a value to string by other means, e.g. when appending it to an existing string: "existing string" + value.

For object, a type check without coercion is performed:

object("foo") // throws
object({})    // returns the object {}
object(null)  // returns null

For ref(T), the same kind of type check without coercion is performed, but additionally the object is required to match T, in a manner described below.

For any, the coercion is a no-op, because any kind of value is acceptable:

any(x) === x

Struct Types

Struct Types are defined using the StructType constructor, which has two overloads:

function StructType(typedFields, name = undefined)
function StructType(baseType, typedFields, name = undefined)

The difference between the two overloads is whether the type inherits from the base Struct Type, Struct, or a more specialized type.

Parameters:

  • baseType - A constructor function whose instances are instanceof Struct, i.e. Struct or a sub-class of Struct.
  • typedFields - An Iterable list of typed field definitions.
  • name [optional] - A string used as the type's name.

Typed field definitions

A typed field definition is an object definining the characteristics of a Struct's typed field. It has the following members:

  • type - A Value Type, specifying the field's type.
  • name [optional] - A string or symbol used as an optional name for the field. If given, an accessor is created that allows reading and, if the field is writable, writing the field using a name in addition to its index.
  • readonly [optional] - A boolean. If true, the field can only be set via the type's constructor and is immutable afterwards.

Struct Type references

Just as other JS objects, Struct Type instances are passed by reference. These references are instances of Value Types. Defining a Struct Type using the StructType constructor actually defines two types: the Struct Type itself, and an accompanying Value Type for references to instances of the Struct Type. Just as other Value Types, it can be used as the type of a Struct Type's fields. This type is exposed as the ref property on the Struct Type's constructor:

const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]);
const Line  = new StructType([{ name: "from", type: Point.ref }, { name: "to", type: Point.ref }]);

Struct Type forward declaration

To enable recursive types, it's possible to declare a Struct Type without defining it. Declaration is done using StructType.declare, which has two overloads:

StructType.declare(name)
StructType.declare(baseType, name)

The first overload declares a Struct Type that extends Struct, the second creates a Struct type that extends the given baseType.

The type can then be defined using its define method:

const LinkedList = StructType.declare("LinkedList");
LinkedList.define([{ name: "next", type: LinkedList.ref }]);

define takes a single parameter, typedFields, and performs the same steps for defining the type's fields as new StructType does.

Instantiation

Instantiating Struct Types

You can create an instance of a Struct Type using the new operator:

const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]);

let from = new Point();
console.log(from[0], from.x); // logs "0, 0"

The resulting object is called a typed object: it will have the fields specified in Line. If no parameters are passed, each field will be initialized to its type's default value:

  • 0 for numeric types
  • '' for string
  • null for object and Struct Type references
  • undefined for any

Any parameters passed are used as initial values for the type's typed fields:

const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]);
const Line  = new StructType([{ name: "from", type: Point.ref }, { name: "to", type: Point.ref }]);

let from = new Point(42, 7);
let line = new Line(from);
console.log(line.from === from, line[0] === line.from, line.from.x); //logs "true, true, 42"

Struct type details

Struct types are specified as a new kind of Built-in Exotic Object. They override some internal methods, much like Integer-Indexed Exotic Objects do.

Memory layout

Struct type objects have the following internal slots:

  • [[FieldTypes]] — A list containing references to each of the type's field's types.

Struct type instance objects have the following internal slots:

  • [[StructType]] — An immutable reference to the instance's type.
  • [[Values]] — A list of the abstract values for all the fields produce by the fields' associated Value Type [[Write]] method

For the Struct type itself, [[FieldTypes]] is set to an empty list.

Typed fields

Struct types have their property-access related internal methods overridden to perform type checking and coercion, based on the internal slots described above.

Reading from typed fields

Reading from a typed field returns the result of converting the internal representation of the field's contents to a value representation, using the field's type's [[Read]] method.

In slightly more detail, a [[Get]] operation on a Struct Type instance O with property key P, and receiver receiver performs the following steps:

  1. If Type(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P) .
    2. If numericIndex is not undefined, then
      1. Let fieldType be O.[[StructType]].[[FieldTypes]][numericIndex].
      2. Let TV be O.[[Values]][numericIndex].
      3. Return ! fieldType.[[Read]](TV).
  2. Return ? OrdinaryGet(O, P, Receiver).

Writing to typed fields

Writing to a typed field stores the result of converting the given value V to an internal representation as the field's contents, using the field's type's [[Write]] method.

In slightly more detail, a [[Set]] operation on a Struct Type instance O with property key P, value V, and receiver receiver performs the following steps:

  1. If Type(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P) .
    2. If numericIndex is not undefined, then
      1. Let fieldType be O.[[StructType]].[[FieldTypes]][numericIndex].
      2. Let TV be ? fieldType.[[Write]](V).
      3. Set O.[[Values]][numericIndex] to TV.
      4. Return V.
  2. Return ? OrdinaryGet(O, P, Receiver).

Immutable typed fields

Typed fields can be marked as readonly, in which case any attempt to write to them after the Struct's construction throws an error.

Named typed fields

When defining a Struct type, its typed fields can optionally be given names. These names can be strings or symbols, with the only restriction being that a string name can't be a valid numeric index, i.e. CanonicalNumericIndexString must return undefined.

If a typed field is named, an accessor with that name will be added to the Struct type's prototype. That accessor simply forwards [[Get]] and [[Set]] operations to the field's numeric index.

Inheritance

Struct types can extend other Struct types. If no base type is given, a Struct type will extend Struct, which is itself a Struct type without any typed fields.

Layout of subtypes

When initializing a Struct Type object O as a subtype of a Struct Type P, the supertype's fields are copied and additional fields are appended by extending O's [[FieldOffsets]] and [[FieldTypes]] internal slots. Instances of the new Struct Type have their [[Values]] buffer extended accordingly.

Type-checking for Struct Type references

The type check for Struct Type references ensures that the given value is an instance of either the specified type itself, or of a sub-type. The applied test has to be stronger than instanceof, since that is easily falsifiable, removing the guarantees around the struct's layout and behavior.

Instead, to facilitate type-checks, Struct type instances have an internal slot [[StructType]], containing a reference to the associated Struct type constructor. Struct type constructors, in turn, have an internal slot [[BaseType]], containing a reference to the type this type extends. For the Struct constructor, the value of this field is null.

Given a value V, the [[Write]] internal method of Struct Type reference types performs the following steps to verify type compatibility for the given value:

  1. Let O be ? ToObject(V).
  2. If O does not have the internal slot [[StructType]], throw a TypeError exception.
  3. Let type be O.[[StructType]].
  4. Repeat, while type is not null
    1. If type is equal to expectedType, return true.
    2. Let type be type.[[BaseType]].

Note: in practice, implementations don't need to, and aren't expected to, perform this expensive test. Well-established fast subtying checks that are equivalent to this check exist.

Overriding named fields

Named fields can be overridden, but some restrictions apply:

  • A field can't change from mutable to immutable and vice-versa.
  • The type of mutable fields is invariant—the field's type must be exactly the same as the overridden field's type.
  • The type of immutable fields is covariant—the field's type can be a subtype of the overridden field's type.

Exotic behavior of Struct type instances

Struct type instances are exotic objects in two ways:

  1. They are immutable prototype exotic objects. See the section on read-only prototypes below.
  2. They override the same set of internal methods as Integer-Indexed Exotic Objects to provide special handling of integer-index property keys, as described above.

Prototypes

Prototypes are set up the same way as for classes: the constructor function form one prototype chain, starting with Struct, and instances have another prototype chain, starting with Struct.prototype.

E.g., for the Struct type Point

  • the [[Prototype]] is set to Struct
  • the [[Prototype]] of instances of Point is set to Point.prototype
  • The [[Prototype]] of Point.prototype is set to Struct.prototype

Read-only [[Prototype]]

The above-described prototype chains are immutable: [[SetPrototypeOf]] throws for all objects on the prototype chains. I.e., all these objects are immutable prototype exotic objects.

Exotic behavior of prototypes

The prototypes of Struct instances are exotic objects in two ways:

  1. As described above, they are immutable prototype exotic objects.
  2. Their overrides of [[DefineOwnProperty]] and [[Set]] prevent shadowing of named typed fields, and defining any integer index property key-named properties.

Combined, these rules enable the same degree of strong reasoning about a Struct type instance's fields whether it's accessed by its integer index property key, or by its—optional—string or symbol name. This also enables easier and more stable optimization in engines, leading to higher and more predictable performance.