Consider a pattern for dictionary arguments which must have at least one member

[domenic]

This is a continuation of #130.

There are a number of places on the platform that end up marking their dictionary arguments as "optional", even though they're not really optional. They just have a requirement that can't be expressed in IDL today: at least one member must be present. Examples:

• DOM's MutationObserverInit (I think... it's a bit complicated)
• Web Share's ShareData
• Media Capture and Stream's MediaStreamConstraints (although they'd need to remove the default = false values, otherwise both members are always present)
• The planned URLPatternInit

I think we might have reached the point where it's worth making this pattern first-class. It would remove a bit of spec boilerplate in each spec, and it would make the IDL less confusing for readers (see e.g. whatwg/dom#332).

I suggest something like

partial interface MutationObserver {
  void observe(Node target, [OneMemberRequired] MutationObserverInit options);
};

I believe this explicitness sidesteps the problems mentioned in #130, where we were worried that people would not properly mark truly-optional trailing dictionaries as optional. You have to really know what you're doing to use this kind of extended attribute.

We could also use a keyword (onememberrequired MutationObserverInit options) for symmetry with optional. In general the dividing line between syntax and extended attributes is kind of blurry in Web IDL; see some discussion in #857 (comment).

[annevk]

It doesn't seem like this would simplify the logic for observe(). And I'm also not sure how it would help getUserMedia() as what it really wants are true or dictionary values, not just any values.

[domenic]

Another instance: WICG/navigation-api#52

[saschanaz]

It doesn't seem like this would simplify the logic for observe().

IMO this one is more about the syntax rather than about simplifying logic. I can imagine an author write an operation with the one-member-required behavior in mind and then get yelled by IDL validator. It also affects TypeScript code generation, BTW.

[domenic]

I'm mostly interested in this for logic-simplification; comments are fine for readers. But I think we have enough cases now where it would simplify the logic.