[swift-evolution] isEqual to replace == Equatable Requirement

[Richard Fox]

Hi all,

I would like to propose changing the Equatable protocol to use isEqual(to:Self)
-> Bool, defined inside of a type to replace the currently used
operator == function,
which is defined outside of the type.
<https://gist.github.com/Nadohs/308603afa65cbbfba07c#reasoning>Reasoning:

   1. Having the conforming function defined inside of the type is more
   intuitive, since in general functions required for conformance are defined
   within the type. It feels like an unnecesary detail for learners of Swift
   to have to stumble through.

The implementation for this would look something like this:

     public protocol Equatable{
       ....

       /// Shortcut for defining `==` function inside type definition.
       @warn_unused_result
       func isEqual(to:Self) -> Bool
     }

     @warn_unused_result
     public func == <T : Equatable>(lhs: T, rhs: T) -> Bool {
         return lhs.isEqual(rhs)
     }

<https://gist.github.com/Nadohs/308603afa65cbbfba07c#impact-on-existing-code>Impact
on Existing Code:

This implementation would break existing code, but could be fixed with a
default protocol extension such as:

     /// Default `isEqual` function to satisfy other types only definiting
     /// `==` for equality.
     public extension Equatable{
         func isEqual(to:Self) -> Bool{
             return self == to
         }
     }

Not adding the default function for isEqual makes more sense to me though,
since it would remove any strict requirement for Equatable conformance and
leave no warning for the loop you would create by implementing neither
isEqual nor ==.

Regards,
Rich Fox
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