Replace RACTupleNil with EXTNil

[jspahrsummers]

I know that RACTupleNil was deliberately chosen over EXTNil at one point, but I don't remember/understand the motivation for it.

EXTNil interoperates with code that expects nil or NSNull, so is more "backwards compatible" in a certain sense. This same property also makes it nicer for things like RACBlockTrampoline, because EXTNil == nil for most intents and purposes.

[joshaber]

It's the "most intents and purposes" part that bothers me. It seems better in my mind to just use a sentinel and real nil than to say "it sends EXTNil which mostly behaves like nil but isn't."

[jspahrsummers]

It's really just identity checks and other methods which reveal it to be an actual object and not truly nil, but there's nothing to prevent mapping it to a real nil, like is done with RACTupleNil.

It's mostly valuable in collections, so you can save and pull out values that actually behave as nil.

[joshaber]

Right, since we're almost always just mapping it to true nil, I'm not sure what the advantage is.

My perspective is this. For consumers, nil will always be better. So as much as we possibly can, we never expose RACTupleNil. But the few times we do expose it, we should be honest about what it is. It's not nil. It can't be used as nil. It's a sentinel. EXTNil blurs that line without much benefit.

[jspahrsummers]

What about RACTupleNil is superior to NSNull, then?

[joshaber]

Nothing, except that it's not NSNull. The meaning of NSNulls isn't well-defined. The meaning of RACTupleNil is.

[jspahrsummers]

I disagree. NSNull is used throughout Cocoa as "nil represented in a collection." In particular, that's how KVC/KVO treats it.

[joshaber]

Perhaps, but users could mean NSNull to indicate something else as well. It might have a different meaning from nil in their use.

[jspahrsummers]

That would be user error, IMO, specifically because NSNull already has special meaning for KVC. Like, if you use it to represent something in a property, for example, -setValuesForKeysWithDictionary: will never be able to set it.

[joshaber]

That's one use of NSNull but AFAIK, there's nothing that says that's the only legitimate use.

RACTupleNil is to RAC as NSNull is to KVC.

[jspahrsummers]

I don't really agree with that, but I think we're at an impasse, and it's not a big deal either way.