Ambiguity when applying multiple mixins and what the Symbol.mixin property of the constructor prototype should be

[qubyte]

The section on Symbol.mixin indicates that a property of the constructor prototpe will be set to the mixin applied. However, it's not obvious what this should be when multiple mixins are applied.

In other words, this snippet:

class C extends A with M1, M2, M3 {}

doesn't seem to mesh with the third line of this:

class C extends A with M {}
Object.getPrototypeOf(C).prototype.hasOwnProperty(Symbol.mixin); // true
Object.getPrototypeOf(C).prototype[Symbol.mixin] === M; // true

I'm a fan of this proposal by the way (I like mixins), and the sibling protocols proposal. Exciting to see them being considered as language features!

[justinfagnani]

Sorry for the delay here!

With multiple mixins, we'll get multiple objects on the prototype chain, each with their own [Symbol.mixin] property.

In the case of:

class C extends A with M1, M2, M3 {}

We'll have the prototype chain:

C' -> AwithM1_M2_M3' -> AwithM1_M2' -> AwithM1 -> A'

(where ' indicates ".prototype")

Each of AwithM1_M2_M3', AwithM1_M2', and AwithM1' will have a [Symbol.mixin] property, pointing to M3, M2, and M1 respectively.

Hope that answers the question. I'll try to clarify in the proposal.