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

Cheyo J. Jimenez cheyo at masters3d.com
Thu Jan 4 18:28:59 CST 2018 

> On Jan 4, 2018, at 3:50 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> 
> On Thu, Jan 4, 2018 at 18:39 Cheyo J. Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
> 
>> On Jan 4, 2018, at 2:55 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> 
>> On Thu, Jan 4, 2018 at 17:15 Cheyo J. Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
>>> On Jan 4, 2018, at 11:53 AM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>> 
>>> 
>>> On Thu, Jan 4, 2018 at 13:46 Cheyo Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
>>> 
>>> 
>>> On Jan 4, 2018, at 10:49 AM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>> 
>>>> I'll admit I hadn't thought of using "unknown default" (or "default unknown"). I don't think that's terrible, but I mildly prefer `unknown case` because it builds on the "pun" that enum elements are also defined using 'case'. If anything hits this part of the switch, it really will be an "unknown case", i.e. a statically-unknown enum element.
>>>> 
>>>> To Cheyo's point, if this were to be a single token I'd probably spell it #unknown, like #available. Then we'd have `case #unknown:` and something that naturally expands to other pattern positions. I found that less aesthetically pleasing, though, and so a context-sensitive keyword seemed like the way to go.
>>>> 
>>>> (For the record, though, I wouldn't describe `case _` as a special case of `default`. They do exactly the same thing, and `_` is a useful pattern in other contexts, so if anything the current `default` should be thought of as syntactic sugar for `case _`.)
>>> 
>>> Can case _ be mixed with unknown case? How can we match all compile time known cases but exclude future cases?
>>> 
>>> What’s your use case for that? That eliminates the possibility of “unknown case” giving you compile-time warnings for subsequently added cases, which was the entire purpose of adding the syntax in the first place.
>> 
>> I was thinking of a generalized `unknown case` pattern but that is out of scope for this proposal. <https://github.com/apple/swift-evolution/pull/777/files#diff-a68dc745ee86d09566b232b6954c5158R321> 
>> 
>> switch excuse {
>>  case .eatenByPet :
>>    //…
>>  unknown case:
>>    // …
>>  case _:
>>    // …
>>  }
>> 
>>> 
>>> Should there be something like `case *` that would capture all currently known cases during compile time? case * and case _ would be the same in exhaustive enums. 
>> 
>> This is why I was suggesting another pattern that only captures known cases at compile time:
>> 
>> switch excuse {
>>  case .eatenByPet :
>>    //…
>>  case * : //  All cases captured at compile time. 
>>    // …
>>  unknown case:
>>    // …
>>  }
>> 
>> Sorry, I don’t understand. However you spell it, what is your use case for this? The stated purpose of “unknown case” is to gain compile-time exhaustiveness testing, but this would not allow for that.
> 
> 
> 
> 
> switch (excuse, notifiedTeacherBeforeDeadline) {
> case (.eatenByPet, true):
>   // …
> case (.thoughtItWasDueNextWeek, true):
>   // …
> case (unknown case, true):
>   // …
> case (_, false):
>   // …
> }
> 
> Im referring to the future direction section in the new PR <https://github.com/jrose-apple/swift-evolution/blob/6061c01fb4a6d742ba7213f46979c9b82891fc14/proposals/0192-non-exhaustive-enums.md#future-directions>. The above example if from there. 
> 
> I am fine with `unknown case` being required to be at the end of the switch for now. 
> 
> I think of `unknown case` as a pattern that only matches unknown cases no matter where on the switch it is.
> 
> This is why I do not think that `default unknown` would work well once `unknown case` can be used a pattern.
> 
> We can start a new thread on this if you’d like. 
> 
> The reason I put forward “default unknown” is precisely because the proposed feature *cannot* be used in a pattern and therefore seems more apt as not a case.
> 
It can not be used in a pattern now but you could in the future if left as `case`. 

> It actually makes it more natural to use in the given example above because “default unknown” could actually be used to provide compile-time exhaustiveness checking for such a tuple pattern, whereas without being able to use “unknown case” in a pattern you can’t write “case (unknown case, _)”.

The way `unknown case` enforces  compile-time exhaustiveness is by only matching unknown cases. The implementation may be more close to default by the virtue of being forced to go at the end of the switch statement now but that should not dictate the user experience. 

> 
> 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). This depends on generalized `unknown case` patterns which is out of scope. I am happy to talk more about this on a different thread when this proposal gets approved. 



> 
> 
>  
> 
>> 
>> 
>> 
>>> 
>>> 
>>>> 
>>>> I'll add these points to the "Alternatives Considered" section in the PR later today.
>>>> 
>>>> Jordan
>>>> 
>>>> 
>>>>> On Jan 3, 2018, at 22:56, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>>>> 
>>>>> As has already been said, “case unknown” is source-breaking because it conflicts with any real cases named “unknown”; “\unknown” looks like a key path but isn’t, and I wonder if it would potentially conflict with existing key paths.
>>>>> 
>>>>> In any case, my point was not to bikeshed the “unknown” part, but to ask whether any consideration had been made to have the feature presented as a flavor of default instead of a flavor of case.
>>>>> 
>>>>> On Wed, Jan 3, 2018 at 23:57 Cheyo Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
>>>>> 
>>>>> 
>>>>> On Jan 3, 2018, at 6:52 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>>> This is a very nice revision. One bikeshedding thought:
>>>>>> 
>>>>>> Since "unknown case" is presented as a special kind of "default", can't be mixed with "default", and can't be used in case patterns, why not "default unknown" (or "unknown default") instead of "unknown case"?
>>>>> 
>>>>> `case _ :` is already a special case of default. 
>>>>> I’d rather have `case unknown :`
>>>>> `unknown case :` is weird because of the order of `case`. 
>>>>> 
>>>>> Another alternative is `case \unknown :`
>>>>> `\unknown` would also allow pattern matching. 
>>>>> 
>>>>> 
>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On Wed, Jan 3, 2018 at 8:05 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>>> On Jan 2, 2018, at 18:07, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>>>>>> 
>>>>>>> [Proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0192-non-exhaustive-enums.md <https://github.com/apple/swift-evolution/blob/master/proposals/0192-non-exhaustive-enums.md>]
>>>>>>> 
>>>>>>> Whew! Thanks for your feedback, everyone. On the lighter side of feedback—naming things—it seems that most people seem to like '@frozen', and that does in fact have the connotations we want it to have. I like it too.
>>>>>>> 
>>>>>>> More seriously, this discussion has convinced me that it's worth including what the proposal discusses as a 'future' case. The key point that swayed me is that this can produce a warning when the switch is missing a case rather than an error, which both provides the necessary compiler feedback to update your code and allows your dependencies to continue compiling when you update to a newer SDK. I know people on both sides won't be 100% satisfied with this, but does it seem like a reasonable compromise?
>>>>>>> 
>>>>>>> The next question is how to spell it. I'm leaning towards `unexpected case:`, which (a) is backwards-compatible, and (b) also handles "private cases", either the fake kind that you can do in C (as described in the proposal), or some real feature we might add to Swift some day. `unknown case:` isn't bad either.
>>>>>>> 
>>>>>>> I too would like to just do `unknown:` or `unexpected:` but that's technically a source-breaking change:
>>>>>>> 
>>>>>>> switch foo {
>>>>>>> case bar:
>>>>>>>   unknown:
>>>>>>>   while baz() {
>>>>>>>     while garply() {
>>>>>>>       if quux() {
>>>>>>>         break unknown
>>>>>>>       }
>>>>>>>     }
>>>>>>>   }
>>>>>>> }
>>>>>>> 
>>>>>>> Another downside of the `unexpected case:` spelling is that it doesn't work as part of a larger pattern. I don't have a good answer for that one, but perhaps it's acceptable for now.
>>>>>>> 
>>>>>>> I'll write up a revision of the proposal soon and make sure the core team gets my recommendation when they discuss the results of the review.
>>>>>>> 
>>>>>>> ---
>>>>>>> 
>>>>>>> I'll respond to a few of the more intricate discussions tomorrow, including the syntax of putting a new declaration inside the enum rather than outside. Thank you again, everyone, and happy new year!
>>>>>> 
>>>>>> I ended up doing these in the opposite order, writing up the new proposal first and not yet responding to the discussion that's further out. You can read my revisions at https://github.com/apple/swift-evolution/pull/777 <https://github.com/apple/swift-evolution/pull/777>.
>>>>>> 
>>>>>> In particular, I want to at least address:
>>>>>> - Dave D and Drew C's points about versioned libraries / linking semantics of modules.
>>>>>> - Jason M's point about migration
>>>>>> and I'll do one more pass over the thread to see if there's anything else I didn't address directly. (That doesn't mean everyone who disagrees, just messages where I think there's more I can do to explain why the proposal is the way it is.)
>>>>>> 
>>>>>> Jordan
>>>>>> 
>>>>>> P.S. Enjoying the Disney references. Thanks, Nevin and Dave. :-)
>>>>>> 
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>>>>>> 
>>>>>> 
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