[swift-evolution] [Review] SE 0192 - Non-Exhaustive Enums

Jonathan Hull jhull at gbis.com
Fri Jan 5 00:56:40 CST 2018

> On Jan 4, 2018, at 10:31 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> On Fri, Jan 5, 2018 at 00:21 Cheyo Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
> On Jan 4, 2018, at 4:37 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> On Thu, Jan 4, 2018 at 19:29 Cheyo J. Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
>> We seem to agree that, by virtue of not supporting use in a pattern and being placed at the end, the feature is a flavor of default. I’m still not sure I understand why you believe it should not be a flavor of default going forward.
>>> You still haven’t answered my question, though—what’s the use case for the feature you propose?
>> My use case would be distinguishing between compile time known cases vs “future only” cases (or unknown cases).
>> I understand that the feature you propose would allow you to make such a distinction, but again, what is your use case for doing so?
> Breaking out early by checking unknown cases first. I admit this is not deal breaker, just a different style I’d like to see supported in the future. 
> I'm still not sure I understand. How can the machine know that it's dealing with an unknown case without first checking if it matches any known case?

I had the same thought as Cheyo.  It isn’t a deal breaker… I like the compromise, but I would prefer it trigger only on an actual unknown case (as opposed to acting like default). I like to break failure cases out at the top when possible. I don’t see any good reason not to support that style.

To answer your question, in the naive sense, it basically is the same question as asking if it is a known case (and then taking the inverse). That doesn’t mean actually checking each case separately though. For example, if the enum cases are internally represented as an unsigned integer, and they are all together in a block, the compiler could simply check that it is greater than the max known value. You could probably even do a bit mask comparison in some cases...

If it is inefficient for some reason, the compiler should be free to rearrange the order of things, as long as it doesn’t change the outcome.

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