Swift Protocol Extension with AssociatedType Constrained to Collection, Can't Use Subscript - swift

Question

I'm trying to write a protocol that conforms to the Collection Protocol, and it has an associatedType - Object and a property object.
protocol DDCDataSource: Collection
{
associatedtype Object
var object: Object {get set}
}
I want to add some default functionality for the case where Object also conforms to the Collection protocol, namely just directly return Object's implementation of these required Collection properties and functions. It seems like it all works except for Collection's requirement for a subscript.
Cannot subscript a value of type 'Self.Object' with an index of type 'Self.Object.Index'
extension DDCDataSource where Object: Collection
{
typealias Index = Object.Index
var startIndex: Object.Index {
get {
return object.startIndex
}
}
var endIndex: Object.Index {
get {
return object.endIndex
}
}
subscript(position: Object.Index) -> Element
{
return object[position]
}
func index(after i: Object.Index) -> Object.Index {
return object.index(after: i)
}
}

Answer 1

Short answer: Change the return type of the subscript method
to Object.Element
subscript(position: Object.Index) -> Object.Element {
return object[position]
}
or add a type alias (in a similar way as you did for the Index type)
typealias Element = Object.Element
subscript(position: Object.Index) -> Element {
return object[position]
}
That makes the code compile and run as expected.
Explanation: The subscript method of Collection is declared as
subscript(position: Self.Index) -> Self.Element { get }
where Self.Index and Self.Element are associated types
of `Collection. With your code
subscript(position: Object.Index) -> Element {
return object[position]
}
the compiler infers Self.Index to be Object.Index, but there
is no relation between Self.Element and Object.Element (which is
returned by object[position]). The error becomes more apparent
if you add an explicit cast:
subscript(position: Object.Index) -> Element {
return object[position] as Element
}
Now the compiler complains
error: 'Self.Object.Element' is not convertible to 'Self.Element'; did you mean to use 'as!' to force downcast?
The correct solution is not the forced cast but to make the compiler
know that Self.Element is Object.Element, by adding a type alias
or by changing the return type
subscript(position: Object.Index) -> Object.Element {
return object[position]
}
so that the compiler infers DDCDataSource.Element to be Object.Element.
Full self-contained example: (Swift 4, Xcode 9 beta 6)
(Note that you can omit the get keyword for read-only computed
properties.)
protocol DDCDataSource: Collection {
associatedtype Object
var object: Object { get set }
}
extension DDCDataSource where Object: Collection {
var startIndex: Object.Index {
return object.startIndex
}
var endIndex: Object.Index {
return object.endIndex
}
subscript(position: Object.Index) -> Object.Element {
return object[position]
}
func index(after i: Object.Index) -> Object.Index {
return object.index(after: i)
}
}
struct MyDataSource: DDCDataSource {
var object = [1, 2, 3]
}
let mds = MyDataSource()
print(mds[1]) // 2
for x in mds { print(x) } // 1 2 3

Answer 2

Firstly, I think you should define Element,
Secondly, you use object[position], Object Conforms To Collection , but it is not of Collection Types . Obviously it is not Array.
As apple
says: array
conforms to CustomDebugStringConvertible / CustomReflectable /
CustomStringConvertible / CVarArg /Decodable / Encodable /
ExpressibleByArrayLiteral /MutableCollection /RandomAccessCollection /
RangeReplaceableCollection
I think extension DDCDataSource where Object: Array is better.
And the element in array shall be Element defined. Just tips.

Answer 3

Try this:
subscript(position:Object.Index) -> Element
{
var element: Element
guard let elementObject = object[position] else {
//handle the case of 'object' being 'nil' and exit the current scope
}
element = elementObject as! Element
}