More nested matching examples

[bterlson]

Especially compositions of different matching types. For example, the following was raised by @marcfawzi on Twitter:

match (p) {
  { name: String, age: Number }: "valid input",
  else: "invalid input"
}

[cesarandreu]

How would you differentiate between p = {name: String, age: Number} and p = {name: '', age: 0}?

[itsMapleLeaf]

As @cesarandreu pointed out, this is problematic, since current semantics imply matching against the String and Number global object literal.

match (p) {
  { name: typeof 'string', age: typeof 'number' }: 'valid input'
  else: 'invalid input'
}

Something like this would be better, I think.

[ljharb]

typeof wouldn't work for non-primitives, though - and for Array and other builtins, it should work cross-realm.

[itsMapleLeaf]

instanceof SomeObject then? Isn't there a thread on extensions to handle these edge cases?

[ljharb]

instanceof doesn't work cross-realm, so it's pretty useless.

[bterlson]

Perhaps I should rename this thread to 'brand checking'. I was talking to @sebmarkbage about this as well. Typeof and instanceof are both valid brand checks in some scenarios, but both also have problems. The problems are exacerbated by the various Realms proposal which will make cross-realm values more common. My feeling is that this proposal should make any brand check an error (eg. Symbol.matches on built-ins throws) until there is a cohesive proposal for brand checking. I don't think this proposal is where to drive that work...

[ljharb]

One possible interaction between @jasnell's proposal, for example, could be that the default matcher on builtins uses the default brand check - I agree this proposal isn't where to drive that work, but I wouldn't want to see one of them land without considering the cross-cutting interaction with the other.

That said, that's a stage 2 concern, so it's way too early to worry much about it.

[7fe]

@cesarandreu As discussed #6 I think using an operator like ~ or the @ symbol makes sense the left(name) and right side(instance of) of the operator would make sense. I have examples here #22

[7fe]

@ljharb it is cross browser supported https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof I'm assuming you mean it doesn't work cross realm meaning it doesn't work between iframes because they are technically different objects yes?

[ljharb]

@limeblack yes, that's what i mean.

[zkat]

Hey y'all! #65 has gotten merged, and a lot of issues have become irrelevant or significantly changed in context. Because of the magnitude of changes and subtle differences in things that seem similar, we've decided to just nuke all existing issues so we can start fresh. Thank you so much for the contributions and discussions and feel free to create new issues if something seems to still be relevant, and link to the original, related issue so we can have a paper trail (but have the benefit of that clean slate anyway).

Note that the new proposal supports arbitrary layers of nesting, so I think this is resolved :)