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

Xiaodi Wu xiaodi.wu at gmail.com
Thu Jan 4 17:50:02 CST 2018 

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 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, _)”.

You still haven’t answered my question, though—what’s the use case for the
feature you propose?


>
>
>
>
>
>>
>>>
>>>
>>> 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
>>>>
>>>>
>
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