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Explainer for value objects

Note: The material in this spec is under discussion.

Motivation

JavaScript today includes a distinction between value types (number, string, etc) and object types. Value types have a number of useful properties:

  • they have no identity and compare by value;
  • they have no associated mutable state;
  • they can be sent across realms;
  • etc.

Unfortunately, the set of value types is correct closed and defined by the spec. It would be nice if it were user extensible.

There are a number of goals:

  • The system should be compatible with the existing value types like string, symbol, and number. Ideally it should be a generalization of how those value types work rather than its own different thing.
  • The engine should be able to freely optimize the representation of these new value types. For example, it should be possible to duplicate them invisibly to the user and/or store a copy on the stack and so forth.
  • ...

The ValueType meta type definition

We introduce the meta type definition ValueType. ValueType can be used to define a new value type definition. It takes the same arguments as StructType, with the exception of a mandatory symbol as the first argument. Unlike StructType, ValueType is a regular function, not a constructor, and hence you do not use the new keyword with it (I'll explain why in the section on prototypes below).

var ColorType = ValueType(Symbol("Color"),
                          {r: uint8, g: uint8, b: uint8, a: uint8});

The type of every field in a value type must itself be a value type or else an exception is thrown.

If you wish to create a value array, that can be done by using a variation on the ValueType constructor:

var ColorArrayType = ValueType(Symbol("Color"), uint8, 4);

Passing a type and number is equivalent to creating a value type with fields named 0...n-1 and all with the same type.

NOTE: Issue #1 proposes to extend this constructor form for fixed-length arrays to non-value types as well.

Creating a value type instance

Instances of a value type are created by invoking the function with a single argument. This argument will be coerced to the value in the same was as with a normal type definition. Hence:

var color = ColorType({r: 22, g: 44, b: 66, a: 88});
color.b == 66

Assigning to fields of a value type

Values are deeply immutable. Therefore, attempting to assign to a field of a value throws an error:

var color = ColorType({r: 22, g: 44, b: 66, a: 88});
color.b = 66 // throws

Prototypes

Instances of value types are not objects. Therefore, like a number or a string, they have no associated prototype. Nonetheless, we would like to be able to attach prototypes to them. To achieve this, we generalize the wrapper mechanism that is used with the existing value types like string etc.

The idea is that there is a per-realm (not global!) registry of value types defined in the engine. This registry is pre-populated with the engine-defined types like strings and numbers (which map to the existing constructors String, Number, etc).

When you apply the . operator to a value, what happens is that the value's type is looked up in the per-realm registry. If an entry C exists, then a wrapper object is constructed whose prototype is C.prototype and execution proceeds as normal.

When you invoke the ValueType function to define a new type, you are in fact submitting a new type to be added to the registry, if it does not already exist. If an equivalent entry already exists, it is returned to you. (More details on this process are in the next section.)

All this machinery together basically means you can attach methods to value types as normal and everything works:

var ColorType = ValueType(Symbol("Color"),
                          {r: uint8, g: uint8, b: uint8, a: uint8});
                           
ColorType.prototype.average = function() {
    (this.r + this.g + this.b) / 3
};

var color = ColorType({r: 22, g: 44, b: 66, a: 88});
color.average() // yields 44

The registry and structural equivalence

I said above that when you invoke the ValueType constructor, you are in fact submitting a new value type to be registered. If a structurally equivalent value type already exists, then that object will be returned to you. This, by the way, is why we do not use the new keyword when creating a value type -- it is not necessarily a new object that is returned.

But when are two types structurally equivalent? The answer is fairly straightforward:

  • Every type definition is structurally equivalent to itself.
  • Two user-defined value-type definitions are structurally equivalent if:
    • they both have the same associated symbol
    • they both have the same field names defined in the same order
    • the types of all their fields are structurally equivalent to one another

Let's go through some examples. First, if you register the same type definition twice, you get back the same object each time:

// Registering the same type twice yields the same object
// the second time:
var ColorSymbol = Symbol("Color");
var ColorType1 = ValueType(ColorSymbol,
                           {r: uint8, g: uint8, b: uint8, a: uint8});
var ColorType2 = ValueType(ColorSymbol,
                           {r: uint8, g: uint8, b: uint8, a: uint8});
ColorType1 === ColorType2

Note that the symbol is very important. If I try to register two value types that have the same field names and field types, but different symbols, then they are not considered equivalent (here, assume that the omitted text ... is the same both times):

// Different symbols:
var ColorType1 = ValueType(Symbol("Color"), ...);
var ColorType2 = ValueType(Symbol("Color"), ...);
ColorType1 !== ColorType2

Similarly, if the symbols are the same but the field names are not, the types are not equivalent:

var ColorSymbol = Symbol("Color");
var ColorType1 = ValueType(ColorSymbol,
                           {r: uint8, g: uint8, b: uint8, a: uint8});
var ColorType2 = ValueType(ColorSymbol,
                           {y: uint16, u: uint16, v: uint16});
ColorType1 !== ColorType2

Design discussion

This setup is designed to permit multiple libraries to be combined into a single JS app without interfering with one another. By default, all libaries will create their own symbols and hence their own distinct sets of value types. This means you can freely add methods to value types that you define without fear of stomping on somebody else's value type.

On the other hand, if libraries wish to interoperate, they can do so via the symbol registry. Similarly, value types that should be equivalent across realms can be achieved using the global symbol registry.

Put another way, ES6 introduced symbols in order to give libraries a flexible means of declaring when they wish to use the same name to refer to the same thing versus using the same name in different ways. Value types build on that.

Comparing values

Two values are consider === if their types are structurally equivalent (as defined above) and their values are (recursively) ===. As a simple case of this definition, if I instantiate two instances of the same value type with the same values, they are ===:

var color = ColorType({r: 22, g: 44, b: 66, a: 88});
color === ColorType({r: 22, g: 44, b: 66, a: 88});
color !== ColorType({r: 88, g: 66, b: 44, a: 22});

In fact, If I define ColorType twice, but using the same Symbol, instances are still equivalent (here, assuming that the stuff I omitted with ... is the same in both cases):

var symbol = Symbol("Color");
var ColorType1 = ValueType(symbol, {...}); // ... == rgba
var ColorType2 = ValueType(symbol, {...});
ColorType1({r:22, ...}) === ColorType2({r: 22, ...}) // ... == same values

However, If I define ColorType twice, but using different Symbol values, then instances are not the same (again, assume that the stuff I omitted with ... is the same in both cases):

var ColorType1 = ValueType(Symbol(...), {...}); // ... == rgba
var ColorType2 = ValueType(Symbol(...), {...});
ColorType1({r:22, ...}) !== ColorType2({r: 22, ...}) // ... == same values

Note that I have only defined === here. It is not clear to me at this moment what the behavior of == ought to be: it might be useful for == to ignore the symbol and only consider field names and types when deciding whether two types are equivalent. Alternatively, if we introduce overloadable operators as described here, then this could potentially be done in user libraries.

The typeof operator

The typeof operator applied to a value type yields its associated symbol. This changes typeof so that it does not always return a string. If this is deemed too controversial, then typeof could return the stringification of the associated symbol, but that seems suboptimal to me because in that case (typeof v === typeof w) has no real meaning (it also permits "forgery" of the return values "string" and so forth). On the other hand, that makes user-defined value types somewhat inconsistent with the existing value types like string.