Operator overloading revisited

Christian Plesner Hansen christian.plesner.hansen at gmail.com
Tue Jun 30 13:37:53 PDT 2009

Sorry about falling behind in a discussion I started.  I'll try to catch up.

> Are you referring to
> <https://mail.mozilla.org/pipermail/es-discuss/2009-January/008535.html>? It
> starts with:
>>> Define a new nominal type named, let's say, "Operable". (I'm not stuck on
>>> this name. Perhaps "Numeric"?) We don't yet know what nominal type system
>>> Harmony will have, so for concreteness, and to separate issues, let's use
>>> "instanceof" as our nominal type test, where only functions whose
> [...suggested restrictions on when this test applies, but all without
> presuming any "type"s beyond what ES5 already supports...]

Yes that's the one.  The main problem I had was with the use of
"instanceof".  However, as I read the proposal again I've become
unsure if I've read something into it that it doesn't say.  The way I
understood it you would implement the + operator for Point instances
something like:

Point.prototype['+'] = function (that) {
  if (that instanceof Point) {
    return new Point(this.x + that.x, this.y + that.y);
  } else {
    return that['reverse+'](this);

// Ditto 'reverse+'

Is that correct?

> ===. Neither do we want
>     Point(3, 5) === Point(3, 5)
> to be false.

Don't we?  I can see that we want Point(3, 5) == Point(3, 5) to be true but ===?

> It seems to me that the Operable-test and value-type issues are orthogonal
> to the core suggestion of your email: whether operator dispatch is based on
> Smalltalk-like double dispatch, in which the left side is asymmetrically in
> charge, or on mutual agreement which is indeed symmetric. The first bullet
> could be changed to your proposal without affecting the rest of the
> value-type issue.

That's probably true, though I don't know how Operable solves the '+'
problem so I couldn't say for sure.

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