> On Dec 31, 2017, at 12:14 PM, Matthew Johnson via swift-evolution 
> <swift-evolution@swift.org> wrote:
> I agree that we need a solution to the problem described.  I also agree that 
> non-exhaustive is most in keeping with the overall design of Swift at module 
> boundaries.  However, I believe this proposal should be modified before being 
> accepted

Thanks for writing this up - you’ve explained a common concern in an 
interesting way:

> This is likely to be a relatively rare need mostly encountered by 3rd party 
> libraries but it will happen.  When it does happen it would be really 
> unfortunate to be forced to use a `default` clause rather than something like 
> a `future` clause which will produce an error when compiled against an SDK 
> where the enum includes cases that are not covered.  I can imagine cases 
> where this catch-all case would need to do something other than abort the 
> program so I do not like the `switch!` suggestion that has been discussed.  
> The programmer should still be responsible for determining the behavior of 
> unknown cases.
> While library authors have a legitimate need to reserve the right to 
> introduce new cases for some enums this need can be met without taking away a 
> useful tool for generating static compiler errors when code does not align 
> with intent (in this case, the intent being to cover all known cases).  
> Switch statements working with these kinds of enums should be required to 
> cover unknown cases but should be able to do so while still being statically 
> checked with regards to known cases.  

I think that this could be the crux of some major confusion, the root of which 
is the difference between source packages and binary packages that are updated 
outside your control (e.g. the OS, or a dynamic library that is updated 
independently of your app like a 3rd party plugin).  Consider:

1) When dealing with independently updated binary packages, your code *has* to 
implement some behavior for unexpected cases if the enum is non-exhaustive.  It 
isn’t acceptable to not handle that case, and it isn’t acceptable to abort 
because then your app will start crashing when a new OS comes out. You have to 
build some sort of fallback into your app.

2) When dealing with a source package that contributes to your app (e.g. 
through SwiftPM), *YOU* control when you update that package, and therefore it 
is entirely reasonable to exhaustively handle enums even if that package owner 
didn’t “intend” for them to be exhaustive.  When *you* chose to update the 
package, you get the “unhandled case” error, and you have maximal “knowability” 
about the package’s behavior.

It seems that your concern stems from the fact that the feature as proposed is 
aligned around module boundaries, and therefore overly punishes source packages 
like #2.  I hope you agree that in case #1, that the feature as proposed is the 
right and only thing we can do: you really do have to handle unknown future 
cases somehow.

If I’m getting this right, then maybe there is a variant of the proposal that 
ties the error/warning behavior to whether or not a module is a source module 
vs a binary module.  The problem with that right now is that we have no 
infrastructure in the language to know this…


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