I’m running into trouble using protocols as types when they have an associated
type. I understand the restriction of using a protocol with an associated type
only as a generic constraint, since our down-casting syntax has no way of
checking the associated types. However, if I’m considering whether an instance
conforms to a sub protocol, which introduces no additional associated types
itself, when the instance is already known to conform to the super-protocol
which introduces the associated type, I believe such behavior is well-defined,
and should be supported. So, I’m asking for help to put together a proposal to
get this added.
Here’s an example:
3 protocols, one base Protocol which declares an associated type.
And 2 sub-protocols.
protocol A {
associatedType V
}
protocol B : A {
func someFunction<V>(V?)->String
}
protocol C : B {
func someOtherFunction<V>()->V?
}
Note that the sub protocols do not add additional associated types.
When I want to use these protocols, using the base protocol as a generic
constraint works fine.
class M {
func doSomething<P, V>(a:P, completion:@escaping(V?)->()) where P : A,
V==P.V {
if let c = a as? C {
c.someOtherFunction(…)
… more code
} else if let b = a as? B {
b.someFunction(…)
… more code
}
}
}
However, when I try to check whether the instance conforms to either of the
sub-protocols, I get an error, saying protocols with associated types can only
be used as generic constraints.
I believe that in this case, since the sub-protocols do not add any additional
associated types, this code should be well-defined, and compilable.
_______________________________________________
swift-evolution mailing list
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
