Often it is the case where one might want to implement a type that provides an interface but has inner components that actually handle the implementation. In those cases, we end up with a lot of boiler-plate code that simply turns around and invokes the on the instance.
Let’s take the example of class clusters:
private protocol _Cluster {
func description() -> String
}
class Cluster: _Cluster {
private var _instance: _Cluster
init(name: String) {
_instance = _ClusterString(name: name)
}
init(value: Int) {
_instance = _ClusterValue(value: value)
}
// this is pure boiler-plate
func description() -> String {
return _instance.description()
}
}
private class _ClusterString: _Cluster {
private var name: String
init(name: String) { self.name = name }
func description() -> String {
return "_ClusterString: \(name)"
}
}
private class _ClusterValue: _Cluster {
private var value: Int
init(value: Int) { self.value = value }
func description() -> String {
return "_ClusterValue: \(value)"
}
}
let s = Cluster(name: "a string")
s.description()
let v = Cluster(value: 12)
v.description()
Now, it would be nice to not have to have to implement the boiler-plate (this example only has a single method, so the savings seem minimal).
class Cluster: _Cluster {
@forward(_Cluster, _instance)
private var _instance: _Cluster
init(name: String) {
_instance = _ClusterString(name: name)
}
init(value: Int) {
_instance = _ClusterValue(value: value)
}
}
The @forward(protocol, instance) attribute lets the compiler know that the _Cluster protocol should be forwarded to the _instance value. The compiler would then generate all of the implementation stubs. Refactoring is also made simple as API changes to _Cluster do not need to be manually reflected on the type.
Another way to solve this problem is with a sufficiently advanced macro system. But that is out-of-scope for v3. However, this seems like it could be a straight-forward enough implementation to support in the mean-time, with an easy path for removal/update if it were to be replaced by a macro system.
-David