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

Thu Jan 4 16:55:29 CST 2018

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.

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