> On Dec 23, 2015, at 3:56 PM, Andrew Duncan <andrewzbo...@gmail.com> wrote:
> 
> 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.
> }

I see. If it's an arbitrary pattern, you can match 'case let r' to bind the 
entire value instead of picking out the payload of the other case. That would 
still be exhaustive.

-Joe

> 
> 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 <jgr...@apple.com> wrote:
>> 
>> 
>>> On Dec 23, 2015, at 10:16 AM, Andrew Duncan via swift-evolution 
>>> <swift-evolution@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
> 

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