[swift-evolution] Revisiting SE-0110

[John McCall]

> On Jun 1, 2017, at 2:39 PM, Pavol Vaskovic <pali at pali.sk> wrote:
> 
> On Thu, Jun 1, 2017 at 8:52 PM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
> I understand that there are developers who dislike SE-0110's impact on certain kinds of functional programming, but that is a very broad complaint that is unlikely to reach consensus or acceptance, especially for Swift 4. 
> 
> The impact of SE-0110 as currently implemented in Swift 4 leads to following migration choice wherever you have a closure that takes tuple argument:
> * concise but obfuscate code ($0.1, ...)
> * readable but verbose code (requiring a ton of boilerplate: intermediate argument, expand signature to include return type, desctructure tuple on new line using let, add return clause)
> 
> Maybe I misunderstood you, but I don't think this is marginal issue affecting only some "developers that dislike the impact on certain kinds of functional programming". 

You're misunderstanding me.  I have explicitly said, several times, that I agree that the impact on tuple destructuring in closures is a serious regression.  There have *also* been objections to losing argument-splat behavior, and while that does negatively affect some functional styles, I think it would be a mistake to try to address that now.

John.

> 
> This is a HUGE regression to the usability of all closure with tuple arguments. I'd wager that is the prevailing consensual opinion of anyone who has experienced this issue in practice. 
> 
> I would go as far as to claim that current implementation of SE-0110 does not conform to the Swift 4's goal of backwards source compatibility.
>  
> In contrast, I think we may be able to gain consensus on a more targeted proposal that just re-admits tuple destructuring in closures, assuming we can find an acceptable implementation.
> 
> From what I understand our options are quite limited because of tight constraints of Swift 4's emphasis on backwards source-compatibility and impending release deadline:
> 
> * effectively revert SE-0110 by always using the codepath for Swift 3 compatibility mode
> * implement full tuple destructuring (unexplored issues with syntax and backwards compatibility)
> 
> (Note that Swift 3 didn't support full tuple destructuring - SR-4738 <https://bugs.swift.org/browse/SR-4738>)
> 
> --Pavol

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