Default equality function of `observables/usePartialSnapshot`

[goce-cz]

The usePartialSnapshot hook accepts equality function as an optional argument, the default value being isShallowEqual.

While the default value makes the hook usable in most of the scenarios out of the box it has a couple of drawbacks:

• it severely differs from how react works
  • React uses Object.is basically everywhere and at some places allows custom functions to be provided
  • this adds a cognitive overhead to any new user of our hook
• doing a isShallowEqual comparison in cases where Object.is would suffice is an overkill
  • while the users can deliberately choose Object.is as the equality function, majority of them won't do it as the default works well
  • this can have a significant negative performance impact as instead of simple inequality detection through Object.is the isShallowEqual has to traverse all the properties

I'm kinda inclined to use Object.is as the default equality function and make the users reach for isShallowEqual manually when needed.

I'd love to hear some opinions here.

[shkarpa]

It looks like you've already made a pretty compelling case for changing the equality function to Object.is in the issue description. I like the idea because if we consider that the comparisons are somehow ordered by complexity Object.is < shallowEqual < deepEqual it makes sense to make the simplest one the default. The only doubting question then is, do we actually have valid use-cases where Object.is suffices? If so, I would proceed with the change right away.

[goce-cz]

The only doubting question then is, do we actually have valid use-cases where Object.is suffices?

The Object.is is IMO sufficient in 90% cases where you are just selecting sub-state. In case of a complex shape, updates usually happen the way that the previous object is just spread and only affected parts are replaced:

return {...prev, affected: newAffected}

so in case we are selecting via:

return state.nonAffected

we are still getting the same instance.

So yeah, I'm in favor of changing this to Object.is.

I would still like to hear an opinion of @Bartunek and @Fezzzi as they are currently the only users of this hook (apart from me).

[Bartunek]

I would change the default to Object.is as well. I agree with what @shkarpa said and it always feels natural to me in similar cases to expect the default be the simplest and most performant option.

[Fezzzi]

I have no objections and agree with all of you 👍

[craig-bishell]

I agree with... every one else. :) I think @Bartunek 's point especially is salient (it always feels natural to me in similar cases to expect the default be the simplest and most performant option), and I agree 100%