> On Sep 12, 2016, at 8:14 AM, Paul Cantrell <cantr...@pobox.com> wrote:
>
>
>> On Aug 11, 2016, at 9:39 PM, Douglas Gregor via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>> FWIW, I'm planning to write a complete proposal for conditional conformances
>> and will start posting drafts once it is far enough along to be useful.
>
> I’m looking forward to that!
>
> I’ll throw a potential use case for this your way, both one of the most
> obvious and one of the nastiest I’ve encountered, in case it helps refine the
> proposal and/or clarify its limits:
>
> // No more runtime exceptions from things buried in
> // dictionaries! Wouldn’t it be nice?
>
> protocol JSONRepresentable { }
>
> // So far so good:
>
> extension String: JSONRepresentable { }
>
> // Hmm, extending protocols is out:
>
> extension Integer: JSONRepresentable { }
> extension FloatingPoint: JSONRepresentable { }
>
> // I can imagine a universe where this works:
>
> extension Optional: JSONRepresentable
> where Wrapped: JSONRepresentable { }
Fine so far.
>
> // ...but this, oh dear:
>
> extension Optional.None: JSONRepresentable { }
This will be ill-formed; Optional.None isn’t a type, it’s part of the value,
and allowing the conformance to be present or absent based on a run-time value
is a massive complication:
func f<T: JSONRepresentable>(_: T) { }
func g(stringOpt: String?) { f(stringOpt) } // whether the conformance
applies or not depends on the run-time value!
>
> // Collection would be better but ... extending protocols again:
>
> extension Array: JSONRepresentable
> where Element: JSONRepresentable { }
>
> extension Dictionary: JSONRepresentable
> where Key == String, Value: JSONRepresentable { }
>
> I imagine this is all a bridge too far for a proposal at this time — and
> maybe a bridge too far for any conditional protocol conformance ever — but it
> seems like it needs addressing.
These last two should be fine.
>> It won't have support for protocols conforming to other protocols, though.
>
> Curious. Why not? (I’ll happily wait for the proposal if you explain it
> there.)
The generics manifesto has a sketch of the reason; I will elaborate in the
proposal.
- Doug
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