On Tue, Jan 2, 2018 at 12:47 AM, Chris Lattner via swift-evolution <
swift-evolution@swift.org> wrote:
> 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.

Yes, absolutely.

> 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.

I agree 100%

> 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.

I take slight exception to the word “only” here: the proposal as written
does not offer compile-time feedback to inform client programmers that
external enums on which they “switch” have new cases, because they are
required to include a “default” clause.

Yes, those client programmers need to handle unknown cases, but they *also*
deserve the compiler’s assistance in identifying when such cases have been
added. I don’t know if a “future” keyword is the best approach, but we
ought to include some way to warn (not error) when compiling a switch
statement that the author intends to be exhaustive-over-all-known-cases
while also handling unknown ones.

> 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…
> -Chris

If we can make that work, this proposal will be much more palatable.

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