[swift-evolution] Enums and Source Compatibility - defaults

[Jordan Rose]

> On Aug 9, 2017, at 22:46, David Hart <david at hartbit.com> wrote:
> 
> 
>> On 10 Aug 2017, at 02:42, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>> 
>> :-) As you've all noted, there are some conflicting concerns for the default:
>> 
>> - Source compatibility: the existing behavior for an unannotated enum is "closed".
>> - Intuition: if you show someone an enum without an explicit annotation, they'll probably expect they can switch over it. (I'm going to say this is why Zach calls it a "sensible default".)
>> - Consistency: switches on an enum in the same module can always be exhaustive, so having it be different across modules is a bit annoying. (But 'public' already acts like this.)
>> 
>> vs.
>> 
>> - Library evolution: the default should promise less, so that you have the opportunity to change it.
>> - Flexibility: you can emulate an exhaustive switch with a non-exhaustive switch using fatalError, but not the other way around.
>> 
>> All of this is why I suggested it be an explicit annotation in either direction, but Matthew brought up the "keyword soup" problem—if you have to write (say) "public finite enum" and "public infinite enum", but would never write "private finite enum" or "private infinite enum", something is redundant here. Still, I'm uncomfortable with the default case being the one that constrains library authors, so at least for binary frameworks (those compiled "with resilience") I would want that to be explicit. That brings us to one more concern: how different should binary frameworks be from source frameworks?
> 
> In terms of intuition and consistency, I think we should really try to learn from the simplicity of public/open:
> 
> * When internal, classes are sub-classable by default for convenience, but can be closed with the final keyword
> * When public, classes are closed to sub-classing for safety, but can be opened up with the open keyword (which implies public).
> 
> If we try to mirror this behaviour (the keywords are just suggestions, not important):
> 
> * When internal, enums are exhaustive by default for convenience, but can be opened-up with the partial keyword
> * When public, enums are non-exhaustive by default for safety, but can be made exhaustive with the exhaustive keyword (which implies public).

This is not a correct understanding of the internal/public distinction for classes, though. From inside a module, a public-but-not-open class is still subclassable, and similarly a public-but-not-"closed" enum will still be exhaustively switched. You don't have to worry about your own module changing out from under you.

Jordan

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