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

[Joe Groff]

> On Dec 23, 2015, at 3:56 PM, Andrew Duncan <andrewzboard at 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 <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
>