[swift-evolution] [Pitch] Guarding on enum values

[Joe Groff]

> On Dec 23, 2015, at 10:16 AM, Andrew Duncan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> In fact, I feel the same way too. I have definite views about indefinite pronouns. When I am teaching, I studiously avoid “it”, “this”, and “that”: at any given instant half the students have wandering minds, and if they miss the referent, they get lost. My old HyperTalk habits must be resurfacing with “it”. :)
> 
> I still think the use case is valuable as a (natural IMHO) generalization of guard, and feel the annoyance of having the bound variable show up three times and outlast the guard, when I don’t want to use or even see it. Brent’s suggestion removes the second objection and alleviates the first; I’ll see that, but ask if we can raise it. The pitch is:
> 
>    guard case let .Succeed(m) = returnsResult() else let r {
>        return r
>    }
> 
> Improvement! The question is: can we reduce this by one or two ‘r’s?

A slight generalization would be to allow for an arbitrary pattern in the `else` clause:

guard case let .Succeed(m) = returnsResult() else case let .Failure(r) {
       return r
}

with the requirement that the "guard" and "else" patterns form an exhaustive match when taken together. That feels nicer than special-case knowledge of two-case enums, though I admit it punishes what's likely to be a common case.

-Joe