[swift-evolution] [Discussion] Generic protocols

Daniel Leping daniel at crossroadlabs.xyz
Tue Dec 6 08:01:25 CST 2016

ConstructibleFromValue<ValueType == V>

Sounds way better to me than the second option considering we can't do just


Because of lack of order of associated types. So IMO either we introduce
order or we use == syntax. Maybe somebody has more ideas of a better syntax?

On Tue, 6 Dec 2016 at 11:33 Anton Zhilin via swift-evolution <
swift-evolution at swift.org> wrote:

> What makes me worry is if this syntax is really the best one possible:
> typealias ConstructibleFrom<V> = ConstructibleFromValue where ValueType == V
> I find it strange that such exact line with typealias and where is
> required. I was hoping for an expression that lets us specialize a protocol
> and use it in-place, like so:
> extension MyType : ConstructibleFromValue<ValueType == V>extension MyType : (ConstructibleFromValue where ValueType == V)
> Also I thought that this topic does not really belong to generalized
> existentials:
> var x: ConstructibleFrom<Float>
> I mean, we specify all the types here, so the compiler should just
> specialize the protocol and use it as a normal existential. The only thing
> that does not allow us to do that now is syntax.
> By comparison, the following typealias does require generalized
> existentials, because some associatedtypes, including Iterator, are not
> specified:
> typealias GenericCollection<E> = Collection where Iterator.Index == Int, Element == E
> It also shows that future generalized-existentials syntax will likely be
> exactly the same as the one we choose now for protocol specialization.
> So we still should think twice before proceeding. We can implement this
> proposal now, but do we want to?
> ‚Äč
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> swift-evolution at swift.org
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