[swift-evolution] Auto-generate op==?

Sat Feb 13 17:43:17 CST 2016

I discussed this recently,  there are two things that might be worth considering at the same time:

There should probably be an attribute so that we can indicate properties that should not be compared. This would also be a useful way to tell the compiler to ignore non-equatable values if we don’t actually care about them in the comparison.
It might be nice now or in future if we could also build upon the auto-generated implementation. For example, by marking properties as ignored like I say, we could then add custom code for these while still letting the compiler auto-generate the simpler cases for us.

> On 13 Feb 2016, at 22:32, Patrick Gili via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Agreed.
> 
> In the property behaviors review, Chris Lattner mentioned that there is a trend to use # to denote compiler synthesis. Thus, this is probably a good alternative.
> 
> Cheers,
> -Patrick
> 
>> On Feb 13, 2016, at 4:37 PM, Developer <devteam.codafi at gmail.com <mailto:devteam.codafi at gmail.com>> wrote:
>> 
>> I'd be in favor of forgoing the keyword addition and just prefixing compiler-derivable protocols with a # or @.  Save `deriving` for later extensions to a future generalized deriving mechanism.
>> 
>> ~Robert Widmann
>> 
>> 2016/02/13 16:29、Patrick Gili via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> のメッセージ:
>> 
>>> I've been playing with the concept in Haskell, and I like the concept. However, I don't like the proposed syntax, as it is not consistent the rest of the language. Maybe something like
>>> 
>>> struct Person : deriving Equatable {
>>>     let age: Int
>>>     let name: String
>>> }
>>> 
>>> or 
>>> 
>>> struct Person : @deriving Equatable {
>>>     let age: Int
>>>     let name: String
>>> }
>>> 
>>> -Patrick
>>> 
>>>> On Feb 13, 2016, at 12:38 PM, Donald Pinckney <djpinckney at ucdavis.edu <mailto:djpinckney at ucdavis.edu>> wrote:
>>>> 
>>>> To make sure I understand correctly, what you are discussing is having == be auto generated in some way for types. The automatic implementation of == would be a member wise logical and operation:
>>>> 
>>>> struct Person {
>>>>    let age: Int
>>>>    let name: String
>>>> } deriving Equatable
>>>> 
>>>> Would also generate:
>>>> func ==(left: Person, right: Person) -> Bool {
>>>>    return left.age == right.age && left.name == right.name;
>>>> }
>>>> 
>>>> The compiler would only do this if all data members implement (or derive) the Equatable protocol, and probably error otherwise.
>>>> 
>>>> That's my interpretation.  If I misunderstood, please correct me.  
>>>> 
>>>> As it stands, I think something like this is a fabulous idea, as I have recently found myself writing lots of member wise equality checks.
>>>> 
>>>> Donald Pinckney
>>>> 
>>>> Sent from my iPhone
>>>> 
>>>> On Feb 13, 2016, at 9:12 AM, Donnacha Oisín Kidney via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> In Haskell, when you declare a datatype, you can follow it with a “deriving” clause, and it will derive several typeclasses (which are Haskell’s equivalent to protocols):
>>>>> 
>>>>> data Person = Person { name :: String, age :: Int } deriving Eq
>>>>> 
>>>>> In Swift, I’d imaging the equivalent would be something like:
>>>>> 
>>>>> struct Person {
>>>>>   let name: String
>>>>>   let age: Int
>>>>> } deriving Equatable
>>>>> 
>>>>>> On 13 Feb 2016, at 17:04, Patrick Gili via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>> 
>>>>>> Not having a lot of experience with Haskell, can you provide an example, so that we can better understand what you're proposing?
>>>>>> 
>>>>>> Cheers,
>>>>>> -Patrick
>>>>>> 
>>>>>>> On Feb 12, 2016, at 3:47 PM, Daniel Tartaglia via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>>> 
>>>>>>> In Haskell, we can mark a data block as deriving from Eq and it will auto-generate the == operator.
>>>>>>> 
>>>>>>> I would like to see Swift auto-generate the == operator if a struct implements Equatable. Obviously, it would only be able to do this if all the structs members implemented Equatable themselves.
>>>>>>> 
>>>>>>> Has this idea already been proposed? I didn’t see it at the github repo…
>>>>>>> 
>>>>>>> Thanks,
>>>>>>> 
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>>>>>> 
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