[swift-evolution] [Draft] Automatically deriving Equatable

[Jan E. Schotsman]

On May 25, 2016, at 9:04 PM, Tony Allevato wrote:

> I was inspired to put together a draft proposal based on an older
> discussion in the Universal Equality, Hashability, and Comparability  
> thread

Thank you for driving this forward.
Equatability through conditional conformance has been discussed a lot,  
but in the end we didn't get very far.

As for implicit or explicit adoption of the default equatability I am  
undecided. If you have to opt in explicitly you will no longer get a  
compiler warning if you forget to write your custom implementation (if  
that is what you intended).

As for omitting properties from equating or hashing,  I find this very  
important. I didn't count my equatable structs, but I think many would  
need a custom implementation, especially the more complex types.
I definitely want the option to include computed properties in the  
equality test. A common scenario would be: omit two stored properties,  
include one computed property instead.

It has also been suggested that property behaviors may be used for this.
I never followed the discussion on these, but here is a test balloon:  
let a property (stored or computed) return an optional Bool when  
compared to the same property of another instance. nil = don't care.  
The property can decide whether to use simple equality (if defined) or  
some struct type-specific equality. More flexible and saves two  
keywords.
I don't know if and when property behaviors are going to happen.  
Switching from attributes to property behaviors would be a thoroughly  
breaking change of course.

Jan E.

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