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

[Andrew Duncan]

More progress! This sounds good, but it looks like what you intend is for r to be the error message in the Result enum type.

enum Result {
case .Fail(String)    // Error message
case .Succeed(MyType) // Something to work with
}

guard case let .Succeed(m) = returnsResult() else case let .Failure(r) {
      return r // Looks like r is bound to the error String. 
               // But maybe you meant r = the entire returnsResult() result.
}

The sort of message-passing error-handling I have in mind is where each method in the call chain returns a full Result enum and each stage checks it for Succeed/Fail, and immediately bails on Fail, returning (propagating) the Result. To be sure, this is sort of what exceptions do under the hood anyway.

My use-case is a recursive descent parser that I want to bail when a syntax error is found. This could happen way deep in the stack of calls. If I consistently return a .Fail(ErrorCode) or .Succeed(ASTNode) from each method, I just pass on the Result in case of .Fail, or use it in case of .Succeed.
 

> On 23 Dec, 2015, at 15:35, Joe Groff <jgroff at apple.com> wrote:
> 
> 
>> On Dec 23, 2015, at 10:16 AM, Andrew Duncan via swift-evolution <swift-evolution at swift.org> wrote:
>> 
> 
> 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