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

Xiaodi Wu xiaodi.wu at gmail.com
Thu Jan 4 18:37:02 CST 2018 

On Thu, Jan 4, 2018 at 19:29 Cheyo J. Jimenez <cheyo at masters3d.com> wrote:

> 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> wrote:
>
>>
>> On Jan 4, 2018, at 2:55 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>
>> On Thu, Jan 4, 2018 at 17:15 Cheyo J. Jimenez <cheyo at masters3d.com>
>> wrote:
>>
>>> On Jan 4, 2018, at 11:53 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>
>>>
>>> On Thu, Jan 4, 2018 at 13:46 Cheyo Jimenez <cheyo at masters3d.com> wrote:
>>>
>>>>
>>>>
>>>> On Jan 4, 2018, at 10:49 AM, Jordan Rose <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.
>

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?

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> 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> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Jan 3, 2018, at 6:52 PM, Xiaodi Wu via swift-evolution <
>>>>> 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> wrote:
>>>>>
>>>>>> On Jan 2, 2018, at 18:07, Jordan Rose <jordan_rose at apple.com> wrote:
>>>>>>
>>>>>> [Proposal:
>>>>>> 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.
>>>>>>
>>>>>> 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. :-)
>>>>>>
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>
>>>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20180105/f1644b35/attachment.html>

More information about the swift-evolution mailing list