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

> 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?

> 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, 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.


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