Extend swift array that hold specific element type

Question

I've been trying to extend a swift array that hold specific type.

protocol Node {
  var nodeId: Int { get }
  var parentId: Int? { get }
  var children: [Node]  { get set }
  var level: Int { get set }
}

extension Array where Element : Node {
/**
unflatten an array of Nodes.
*/
func unflattenNodes()-> [Node] {

    func unflatten(inout parent: Node?)-> [Node] {

        var topNodes = [Node]()
        let children = self.filter{ (child: Node) in return child.parentId == parent?.nodeId ?? 0 }

        if !children.isEmpty {

            if var parent = parent {
                for var child in children {
                    child.level = parent.level + 1
                }
                parent.children = children
            }
            else {
                topNodes = children
            }

            for child in children {
                var optChild = Optional(child)
                unflatten(&optChild)
            }
        }
        return topNodes;
    }

      var optChild: Node? = nil
      return unflatten(&optChild)
 }
}

This code above won't be compiled because 'Node' is not subtype of 'Element', even though I'm extending Array of element "Node". How can I archive what I'm trying to do here in swift? I'm trying to add an instance method to [Node] to unflatten self.

Have you tried stackoverflow.com/questions/31806324/… ?

Paulw11
– Paulw11

2015-10-05 20:39:23 +00:00
Commented Oct 5, 2015 at 20:39 — Paulw11
– Paulw11, Commented Oct 5, 2015 at 20:39
I just tried and it worked. Thanks!

kazuochi
– kazuochi

2015-10-05 23:28:36 +00:00
Commented Oct 5, 2015 at 23:28 — kazuochi
– kazuochi, Commented Oct 5, 2015 at 23:28

Kametrixom · Accepted Answer · 2015-10-05 20:40:53Z

1

The problem is that the protocol Node doesn't conform to itself: Node is not of type Node (as contradicting as it sounds), because a protocol isn't a concrete type, it's just a requirement for the conforming type.

That said, you need to replace every instance of Node in your code with Element, then it should work (can't test it right now). This is because again, you're actually not handling some value of type Node, but rather a value of a type that conforms to Node (Element in your case)

Also you made a few other mistakes in your code, I'll leave that to someone else to correct as I'm out of time.

answered Oct 5, 2015 at 20:40

Kametrixom

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JeremyP · Accepted Answer · 2015-10-05 20:56:53Z

Right, I've plugged the code into Playground and I found three issues. The first is the one mentioned by Kametrixom which is that you need to use Element as the type everywhere in unflattenNodes. When you have fixed that and got rid of all the parent: in the calls to unflatten, you'll be left with one error

parent.children = children
error: cannot assign a value of type '[Element]' to a value of type '[Node]'
                parent.children = children
                                  ^

This is caused by the fact that the children property in the protocol is defined as [Node] and Swift can't match it to an array of Element

So you need to change the protocol

protocol Node {
    var nodeId: Int { get }
    var parentId: Int? { get }
    var children: [Self]  { get set }
    //            ^^^^^^ Here
    var level: Int { get set }
}

The children property is no longer an array of Node but an array of whatever type is conforming to the protocol, in your case, Element

Daniel Klöck · Accepted Answer · 2016-07-13 11:48:25Z

1

Updating extension Array where Element : Node to extension CollectionType where Generator.Element == Node resolved the error.

edited Jul 13, 2016 at 11:48

Daniel Klöck

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answered Oct 5, 2015 at 23:30

kazuochi

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