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

Xiaodi Wu xiaodi.wu at gmail.com
Tue Jan 2 18:38:17 CST 2018

On Tue, Jan 2, 2018 at 4:31 PM, Jonathan Hull <jhull at gbis.com> wrote:

> I think there are a couple of different definitions running around, and
> that is confusing things.
> In my mind, ‘unexpected:’ catches only cases which were unknown at compile
> time. Adding cases to an enum *should* be a source-breaking change. That is
> the whole point of this.  We should have to update the switch (either by
> handling new case explicitly, or by adding default) before successfully
> compiling.  What ‘unexpected:’ protects against are changes to a linked
> binary (e.g. iOS) that are now vending cases we didn’t know about when we
> were compiled.
> I’ll say it again… framing this idea as one of exhaustiveness is really
> confusing.  Enums should just always be exhaustive in swift.  There may be
> cases where we need to use ‘unexpected:’ to handle unexpected/future cases
> exhaustively.  If we annotate an enum as @frozen, then we won’t need to do
> that to be exhaustive because we know it won’t change out from under us.
> Always exhaustive. Much less confusing…
> Thanks,
> Jon

I think, then, you fundamentally disagree with the starting premise of the
proposal, which is specifically the addition of nonexhaustive enums to the
language, and making them the default sort of enum so that adding cases *is
not* a source-breaking change. If your whole purpose is to change the
proposal so that adding cases will _always_ be a source-breaking change and
enums are _never_ nonexhaustive, then I'm not sure how to proceed in the
discussion as we're working towards diametrically opposite goals.

On Jan 2, 2018, at 1:41 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Tue, Jan 2, 2018 at 3:27 PM, Kevin Nattinger <swift at nattinger.net>
> wrote:
>> [...]
>> in what other circumstances do we insist that the compiler inform the end
>>> user about future additions to the API at compile time?
>>> This isn’t a request for the compiler to inform the user about future
>>> additions to an API.  It is a request to validate the compiler’s knowledge
>>> of the *current* state of an API with the *current* state of the source
>>> code.
>> Well, it's of course impossible to inform the user about future
>> additions, so that's poorly phrased on my part. It's about the compiler
>> informing the end user about *new* additions, part of the *current* state
>> of the API, that have cropped up since the user last revised the code when
>> the API was in a *previous* state (or, indistinguishably, members of which
>> a user is unaware regardless of the temporal sequence of when such members
>> were added). In what other circumstances do we insist that the compiler
>> perform this service?
>> Enums. That's literally how they work today. You are arguing in favor of
>> actively removing compiler-aided correctness.
>> There's also protocol requirements
> No, that's now how enums work today, and it's not how protocol
> requirements work today. Enums today are all semantically exhaustive; if a
> case is added in a later version of a library, it's semantically a
> *different* enum type that happens to share the same name. Not considering
> all the cases of an exhaustive enum is an _error_, not a _warning_, because
> there is no basis on which to proceed. This will not change with the
> proposal. Likewise, adding a protocol requirement without a default
> implementation is source-breaking. The result is a compiler _error_.
> The question is, what non-source breaking API additions today cause the
> compiler to inform the end user of such additions? The answer is: none
> whatsoever. Not new methods or properties on a type, not even new protocol
> requirements that have a default implementation.
> and, arguably, deprecated methods with a proper message ("use foo
>> instead").
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