extension Array {
func filterWithId<T where T : Idable>(id : String) -> [T] {
...
}
}
defines a generic method filterWithId() where the generic
placeholder T is restricted to be Idable. But that definition introduces a local placeholder T
which is completely unrelated to the array element type T
(and hides that in the scope of the method).
So you have not specified that the array elements must conform
to Idable, and that is the reason why you cannot call
self.filter() { ... } with a closure which expects the elements
to be Idable.
As of Swift 2 / Xcode 7 beta 2, you can define extension methods on a generic type which are more restrictive on the template
(compare Array extension to remove object by value for a very similar issue):
extension Array where Element : Idable {
func filterWithId(id : String) -> [Element] {
return self.filter { (item) -> Bool in
return item.id == id
}
}
}
Alternatively, you can define a protocol extension method:
extension SequenceType where Generator.Element : Idable {
func filterWithId(id : String) -> [Generator.Element] {
return self.filter { (item) -> Bool in
return item.id == id
}
}
}
Then filterWithId() is available to all types conforming
to SequenceType (in particular to Array) if the sequence element
type conforms to Idable.
In Swift 3 this would be
extension Sequence where Iterator.Element : Idable {
func filterWithId(id : String) -> [Iterator.Element] {
return self.filter { (item) -> Bool in
return item.id == id
}
}
}