throw on non-Symbols

[michaelficarra]

I don't think these predicates should produce a Boolean result for non-Symbols. Receiving a false implies that we could answer the question but the question is, in fact, unanswerable.

[ljharb]

Predicate functions don't typically throw on any input - if it's not a symbol, then logically it's not a well-known symbol - how is the question unanswerable?

[ljharb]

Some existing precedent is Array.isArray, which never throws for any input.

(If we went with prototype methods, or accessors, then I would definitely expect throwing if the receiver isn't a symbol)

[michaelficarra]

Ah, I was thinking of the question "is this Symbol well-known", not "is this a Symbol that is well-known" (probably because the name is just isWellKnown instead of isWellKnownSymbol). You're right that the former question is probably more expected of a prototype method. This is fine, then. Personally, I would probably rename it to isWellKnownSymbol to make the implication stronger.

[ljharb]

It already is named that imo, since it’s on “Symbol” - its common use will have the words “Symbol is well known”, which would make that name redundant. We can certainly discuss it in plenary tho.

[michaelficarra]

The "Symbol is well-known" reading is exactly why I thought it should throw for non-Symbols.

I think the difference here is whether the "is" is followed by a noun or an adjective. Array.isArray is fine. Array.isEmpty would need to throw on non-Arrays. However, Array.isEmptyArray could just return false for non-Arrays.

[ljharb]

I see your logic, but i don’t agree with it; a predicate should never throw. I’d expect Array.isEmpty to have an implied “is array” as well.

It could return undefined instead of false in these cases, though, since that’s falsy.

[ljharb]

The proposal achieved stage 2 today. Are you content with returning false for non-symbols, or is this a strongly held opinion?

[michaelficarra]

I think this is something I'd like to hear opinions on from the broader committee.

[ljharb]

Gotcha, then I should have asked that today. I'll see if I can squeeze in an overflow item.

[michaelficarra]

@ljharb You can try sharing the thread on Matrix first or something.

[ljharb]

good call, i'll do both!

[ljharb]

Rough consensus in plenary today was to keep them non-throwing, but to rename them to Symbol.isWellKnownSymbol and Symbol.isRegisteredSymbol.