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

[Cheyo J. Jimenez]

> On Jan 5, 2018, at 4:42 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> 
> On Fri, Jan 5, 2018 at 03:11 Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
>> On Jan 4, 2018, at 11:02 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> 
>> On Fri, Jan 5, 2018 at 01:56 Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
>>> On Jan 4, 2018, at 10:31 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto: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...
>> 
>> These are obvious optimizations, but why does this require new syntax?
> 
> I am not sure I understand what you are asking. There isn’t additional syntax.  We are just arguing over the name + behavior of ‘unexpected:’.  You want it to behave like ‘default’ and I am saying that stops the use case I mention above.
> 
> Cheyo said he wants “unexpected case” to work in pattern matching, as well as a new “case *” that is distinct from “case _”. This is additional syntax. When asked what the use case was for these suggestions, he said he wants to distinguish between known and unknown cases at the beginning of the switch.
> 
> 
> 
>> What do you gain from writing the unknown case first?
> 
> I know where to look for the failure cases.  I also tend put a bunch of guard statements near the beginning of a function.  It is just a programming style.
> 
> With my behavior of ‘unexpected:’ you can put it wherever you want.  Why limit that by forcing it to go at the end?
> 
> As pointed out earlier (by one of the core team members, I think), meaningful resilience would mean that the unexpected or unknown case should have useful work executed at runtime; the intention is that the user *shouldn’t* be treating it as a runtime “failure case,” as it effectively makes adding an enum case a change that is incompatible with existing binaries (i.e., not terribly resilient).
> 
> As you and I seem to agree, reaching an unexpected case requires at least notionally considering which cases are expected in the first place. This is the dictionary definition of a default, and Swift usage is to put the default case at the end of a switch statement. Adding new syntax as Cheyo suggests to enable putting it elsewhere, merely for “style,” doesn’t seem to pass the bar for new syntax, nor is it consistent with existing Swift usage.

I agree with the new syntax suggestion is premature. I would hope that the proposed runtime "Deriving collections of enum cases” <https://github.com/apple/swift-evolution/pull/114/files> function also has a compile time version which we can then use to match all the known cases at compile time. I am happy with SE 0192 as it stands now. 


> 
>> Isn't this basically the same thing as asking for the ability to write the default case first, a frequently suggested and rejected syntax addition?
> 
> No.  I don’t think I have ever heard that asked for,
> 
> It has been asked for more than once on this list.
> 
> but putting default in a different place has a different meaning.  The way I read a switch statement anyway is that it tries each case until it find one that matches.  Default matches everything, so it has to go at the end (since it will always match and nothing afterwards will be tried).
> 
> Having ‘unexpected:’ also match known/expected cases is problematic as a mental model.  I think that is just an artifact of the original proposal using default.  There is no reason 'unexpected:’ should have to handle known cases as well… let’s just have it trigger on unexpected ones.
> 
> I don’t think anyone is proposing that the unexpected or unknown case should also match expected cases. Where did you see such a suggestion?
> 
> Thanks,
> Jon

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