Implementation selection

[TitanNano]

I read the proposal a couple of times now, but I'm still failing to see who implementation selection is going to work.

Let's say I create my own ToString protocol:

Protocol ToString {
    symbol;

    toString() {
       return this[ToString.symbol];
    } 
}

const myObject = {
    [ToString.symbol]: 'myObject',
};

console.log(myObject.toString());

How is the implementation of .toString() being selected? Will this run Object.prototype.toString() or ToString.toString()?

[ljharb]

In your case, myObject doesn’t fully implement the protocol, so it’ll fall back to the Object.prototype method. If it did fully implement it, it’d use the ToString.toString method.

[TitanNano]

@ljharb you are right, I forgot

Protocol.implements(myObject, ToString);

But what if I have two competing Protocols which both implement toString()? And how would I select the original Object.prototype.toString() implementation?

[ljharb]

Generally speaking, there'd only be two options: to have the last one win, or to throw. I assume that rather than silently doing the wrong thing, this proposal would choose to throw, so that you couldn't mix competing protocols.

If a protocol you want to use provides a toString implementation, then either you use its toString, or you don't use that protocol.

[TitanNano]

@ljharb I see, so a library can not use its own protocol internally without affecting the consuming application...

After I opened this, I saw PR #32 which appears to propose using myObject[ToString].toString(), that would avoid this issue, but also feels clunky.