I wouldn’t keep it that narrow - monadic types like Optional also benefit from
variance:
func p(_ data:Any?) {
if data != nil {
data.map { print($0) }
}
}
var a:String? = "foo"
p(a)
// -> “foo"
-DW
> On Dec 9, 2016, at 12:24 PM, Hooman Mehr via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> For the specific case of custom collections, I think it is worth providing a
> protocol as Doug noted before.
>
> Quoting Doug Gregor (1/13/16, thread: "Make generics covariant and add
> generics to protocols”):
>> Swift’s value-semantic collections are covariant in their generic
>> parameters, which we do through some fairly tight coupling between the
>> compiler and standard library. From a theoretical standpoint, I’m very happy
>> with the way value-semantic collections provide subtyping and mutation while
>> maintaining soundness (== no runtime checks needed), and for me I would
>> consider it “enough” if we were to formalize that compiler/collection type
>> interaction with some kind of protocol so other collection types could opt
>> in to subtyping, because I don’t think variance—as a language
>> feature—carries its weight outside of the fairly narrow collection-subtyping
>> cases.
> (Emphasis mine)
>
> I also agree with Doug and you that variance does not carry its weight
> outside of collection-subtyping cases.
-DW_______________________________________________
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