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

[Thorsten Seitz]

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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