[swift-evolution] [Accepted] SE-0185 - Synthesizing Equatable and Hashable conformance

[Haravikk]

> On 19 Aug 2017, at 19:46, Daryle Walker <darylew at mac.com> wrote:
> 
>> On Aug 19, 2017, at 7:06 AM, Haravikk via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On 19 Aug 2017, at 11:44, Tino Heth <2th at gmx.de <mailto:2th at gmx.de>> wrote:
>>>> Am 17.08.2017 um 20:11 schrieb Haravikk via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>> For me the whole point of a basic protocol is that it forces me to implement some requirements in order to conform; I can throw a bunch of protocols onto a type and know that it won't compile until I've finished it, developers get distracted, leave things unfinished to go back to later, make typos etc. etc. To me declaring a conformance is a declaration of "my type will meet the requirements for this make, sure I do it", not "please, please use some magic to do this for me"; there needs to be a clear difference between the two.
>>> 
>>> My conclusion isn't as pessimistic as yours, but I share your objections: Mixing a normal feature (protocols) with compiler magic doesn't feel right to me — wether it's Equatable, Hashable, Codable or Error.
>>> It's two different concepts with a shared name*, so I think even AutoEquatable wouldn't be the right solution, and something like #Equatable would be a much better indicator for what is happening.
>>> 
>>> Besides that specific concern, I can't fight the feeling that the evolution process doesn't work well for proposals like this:
>>> It's a feature that many people just want to have as soon as possible, and concerns regarding the long-term effects are more or less washed away with eagerness.
>>> 
>>> - Tino
>>> 
>>> * for the same reason, I have big concerns whenever someone proposes to blur the line between tuples and arrays
>> 
>> Agreed. To be clear though; in spite of my pessimism this is a feature that I do want, but I would rather not have it at all than have it implemented in a way that hides bugs and sets a horrible precedent for the future.
> 
> I tried to make a split thread for this, but would you object to synthesized conformance if we had to explicitly add a command within the definition block to trigger the synthesis? If we add strong type-aliases, we could reuse the directive to copy an interface (method, inner type, property, or conformed-to protocol) from the underlying type to the current type for synthesis too. The only problem would be backward compatibility; once added, we would urge users to explicitly list “publish Equatable” for synthesis, but what about code that already uses the implicit version (since this feature will probably be released for at least one Swift version by the time strong type-aliases happen), do we force users to change their code?

I would rather no code at all use the implicit version; one of my points is that it's not something that's easily changed after the fact, which is why it needs to be done correctly now.

I'm open to any method that makes opting in to the synthesised conformance explicit; I still think a specifically named protocol is the simplest, but I'm not married to that as a solution; attributes, keywords etc. are all fine too, whatever is the easiest way to opt-in to the behaviour explicitly without ambiguity. I'm not 100% sure exactly what you mean by "add a command within the definition block", or is an attribute/keyword what you meant?
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