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>> On Jan 1, 2018, at 11:47 PM, Chris Lattner <> wrote:
>> On Dec 31, 2017, at 12:14 PM, Matthew Johnson via swift-evolution 
>> <> 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…

Hi Chris, thanks for your reply.

The concern you describe isn’t exactly what I was describing but it is related. 
 John McCall recently posted a sketch of a solution to the concern you describe 
which looked great to me.  I don’t have time to look up the link this morning 
but I think it was in this review thread.

The actual concern I am describing is where a 3rd party library (or app) wants 
to switch over a non-exhaustive enum provided by a module that is a binary (not 
source) dependency.  The author of the 3rd party library may have a legitimate 
reason to switch over an enum despite the author of the binary module reserving 
the right to add additional cases.  

When this circumstance arises they will do it using the tools provided by the 
language.  Regardless of the final language solution they obviously need to 
cover unknown cases - their library could be shipping on a device which 
receives an update to the binary dependency that contains a new case.  I agree 
with you that a language-defined crash is not appropriate.  The author of the 
switch must take responsibility for the behavior of unknown cases.  

I am arguing that these “pseudo-exhaustive” switch statements will exist in the 
wild.  The crucial point of contention is whether or not the language provides 
assistance to the author of the 3rd party library in updating their library 
when the enum provided by the binary dependency changes.  Is the author forced 
to use a `default` case which turns of exhaustiveness checking?  Or are they 
able to use an alternative mechanism for handling unknown cases which does not 
turn off exhaustiveness checking - all statically known cases must be covered.  
The most common example of such a mechanism is the `future` (or perhaps 
`unknown`) case which would only be used for cases that are not statically 

This facility will of course help authors of these switch statements make the 
necessary updates as the enum vended by the binary dependency changes.  It will 
also help alert authors of apps that depend on that 3rd party library (which 
will usually be a source dependency).  If the author of the app attempts to 
rebuild the dependency against a new SDK with added cases the library will fail 
to build, alerting the user that they should update the 3rd party library.

My position is that if there are reasonable use cases for these kinds of 
“pseudo-exhaustive” switches then the language should provide exhaustiveness 
checking of statically known cases via some mechanism that authors can opt-in 
to using.  It’s ok with me if this is a relatively esoteric feature.  It won’t 
be commonly needed, but when it is necessary it will provide significant value.

IIRC there were some reasonable examples of these kinds of switches posted in 
earlier threads on this topic.  I don’t have time to look those up right now 
either but it would be good for the core team to be aware of them before making 
a final decision.

I am only aware of two arguments against this kind of “pseudo-exhaustive” 
switch.  One is that users should not attempt to switch over an enum that a 
library author does not intend to be exhaustive (i.e. it is an “input-only” 
enum).  The other is that a `future` or `unknown` case is not testable.

The first argument is a moral one which I believe should not carry much weight 
relative to concrete, pragmatic counter-examples such as those that (IIRC) were 
provided on this list in the past.

The second argument doesn’t make sense to me: as I noted in my review post, the 
code path is equally untestable when a `default` case is used, but all 
statically known cases are matched in earlier patterns.  The testability 
problem of non-exhaustive enums is orthogonal to the issue of language support 
for “pseudo-exhaustive” switches.

This is the line of reasoning which leads me to conclude that we should dig up 
the concrete examples which have been provided and evaluate them for merit.  If 
we can’t discard them as a bad coding practice for which a better solution is 
available then we should strongly consider providing language support for these 
use cases.  The existence of such use cases should also motivate a solution to 
the testability problem.

- Matthew

> -Chris
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