I would like extend Array
to add conformance to a new protocol — but only for arrays whose elements themselves conform to a specific protocol.
More generally, I’d like to have types (whether protocols or concrete types) with type parameters implement a protocol only when the type parameters match certain constraints.
As of Swift 2.0, this appears to be impossible. Is there a way I’m missing?
Example
Suppose we have the Friendly
protocol:
protocol Friendly {
func sayHi()
}
We can extend existing types to implement it:
extension String: Friendly {
func sayHi() {
print("Greetings from \(self)!")
}
}
"Sally".sayHi()
We can also extend Array
to implement sayHi()
when its elements are all Friendly
:
extension Array where Element: Friendly {
func sayHi() {
for elem in self {
elem.sayHi()
}
}
}
["Sally", "Fred"].sayHi()
At this point, the type [Friendly]
should itself implement Friendly
, since it meets the protocol’s requirements. However, this code doesn’t compile:
extension Array: Friendly where Element: Friendly {
func sayHi() {
for elem in self {
elem.sayHi()
}
}
}
The error message is “extension of type 'Array' with constraints cannot have an inheritance clause,” which seems to shut the door definitively on that direct approach.
Is there an indirect workaround? Some clever trick I can use? Perhaps there’s a way that involves extending SequenceType
instead of Array
?
A working solution would make this code compile:
let friendly: Friendly = ["Foo", "Bar"]
Update: This has landed in Swift 4.1, and it is a thing of beauty!
The extension Array: Friendly where Element: Friendly
example now compiles as given in the original question.