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

[Félix Cloutier]

Wait, no, there's a problem with that. You can't use `r` in the guard scope because `returnsResult()` might not have succeeded.

Félix

> Le 24 déc. 2015 à 09:37:36, Félix Cloutier via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> I like that it's consistent with the if syntax (even though I don't really like the if syntax) and that there's no dangling parts after the else.
> 
> Félix
> 
>> Le 24 déc. 2015 à 06:29:17, Thorsten Seitz via swift-evolution <swift-evolution at swift.org> a écrit :
>> 
>> What do you think of
>> 
>> guard let r = returnsResult(), case let .Succeed(m) = r else {
>>       return r 
>> }
>> 
>> Which binds r only within the scope of the guard as desired.
>> 
>> Written in multiple lines
>> 
>> guard 
>>       let r = returnsResult(), 
>>       case let .Succeed(m) = r 
>> else {
>>       return r 
>> }
>> 
>> -Thorsten 
>> 
>>> Am 24.12.2015 um 01:02 schrieb Andrew Duncan via swift-evolution <swift-evolution at swift.org>:
>>> 
>>> Yes, which would revert to Brent’s suggestion. But you have generalized it in a very compatible way.
>>> 
>>> As I read somewhere, improving programming languages comes from removing limitations rather than adding features. I intend for this Pitch to be the former, although it does kind of look like the latter.
>>> 
>>>> On 23 Dec, 2015, at 15:58, Joe Groff <jgroff at apple.com> wrote:
>>>> 
>>>> 
>>>>> 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
>>> 
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