> On 10 Aug 2017, at 19:19, Jordan Rose <jordan_r...@apple.com> wrote:
>>> On Aug 9, 2017, at 22:46, David Hart <da...@hartbit.com> wrote:
>>> On 10 Aug 2017, at 02:42, Jordan Rose <jordan_r...@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.

Correct. Thanks for the clarification! But you still agree with the argument, 
right? Convenience for same module enums (exhaustive by default), safety for 
clients of the module (not-exhaustive when public from outside the module), 
with the option to be explicit? And what do you think of the idea of having the 
« exhaustiveness » modifier imply public, like open does?

> Jordan
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