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

Xiaodi Wu xiaodi.wu at gmail.com
Fri Jan 5 06:42:32 CST 2018

On Fri, Jan 5, 2018 at 03:11 Jonathan Hull <jhull at gbis.com> wrote:

> On Jan 4, 2018, at 11:02 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Fri, Jan 5, 2018 at 01:56 Jonathan Hull <jhull at gbis.com> wrote:
>> On Jan 4, 2018, at 10:31 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> On Fri, Jan 5, 2018 at 00:21 Cheyo Jimenez <cheyo at masters3d.com> wrote:
>>> On Jan 4, 2018, at 4:37 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> On Thu, Jan 4, 2018 at 19:29 Cheyo J. Jimenez <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

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?

> Jon
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