[swift-evolution] Retroactive protocol inheritance

[Tony Allevato]

This is mentioned in the Generics Manifesto as "Conditional Conformances
via Protocol Extensions": <
https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances-via-protocol-extensions
>

While it would be a very powerful and useful feature, it's apparently also
very difficult unfortunately :(  One of the biggest issues with
implementing it would be the effects that is has on dynamic type checking
for protocols, and there's a good discussion about it on the list that's
better than anything I could write: <
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160905/027032.html
>

On Fri, Sep 22, 2017 at 1:53 PM Nevin Brackett-Rozinsky via swift-evolution
<swift-evolution at swift.org> wrote:

> With Swift 4 out, I’ve started using the numeric protocols quite a bit,
> and they are great!
>
> One thing I find myself wishing for is a protocol that extends Numeric
> while also allowing division—a Field protocol, if you will. I have
> implemented several algorithms generically over FloatingPoint because they
> need division, which means they aren’t available for, eg., a Rational type.
>
> Absent a Field protocol in the standard library, I can create one of my
> own:
>
> protocol Field: Numeric {
>   static func / (lhs: Self, rhs: Self) -> Self
> }
>
> And I can extend Float and Double and Float80 to conform easily enough.
> However, I’d really like to write,
>
> extension FloatingPoint: Field {}
>
> and make every type which conforms to FloatingPoint (such as a third-party
> Complex type), also conform to Field.
>
> Is this potentially feasible? Would other people find it useful?
>
> Nevin
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