Protocols and associated types in arrays

Question

I have a protocol Node:

protocol Node {
   var parent: Node?
   var children: [Node]
}

which is implemented by classes:

class TreeNode: Node {
   var parent: Node?
   var children: [Node]
}

But this poses a problem as accessing the parent in TreeNode now gives me a Node, and I want to do TreeNode specific operations on them. So I want to change the protocol to:

protocol Node {
   associatedtype T: Node

   var parent: T?
   var children: [T]
}

Which let's me define the class as:

class TreeNode: Node {
   var parent: TreeNode?
   var children: [TreeNode]
}

Great! But there's a catch. If I want to write a helper method for Node that deals with arrays:

func getReversedChildren<T: Node>(node: T) -> [T] {
   return node.children.reversed()
}

The compiler fails with the error: Cannot convert return expression of type 'ReversedCollection<[T.T]>' to return type '[T]'

From what I can gather about this issue, I need to achieve a type-erasure mechanism to support this architecture. But how can this be done on my example?

How is ArcNode defined ?

claude31
– claude31

2019-01-19 13:31:07 +00:00
Commented Jan 19, 2019 at 13:31 — claude31
– claude31, Commented Jan 19, 2019 at 13:31
@claude31 Typo, should be Node. Updated question

Oskar
– Oskar

2019-01-19 15:56:27 +00:00
Commented Jan 19, 2019 at 15:56 — Oskar
– Oskar, Commented Jan 19, 2019 at 15:56

Martin R · Accepted Answer · 2019-01-19 13:34:59Z

3

You probably want the parent and children of a node to be of the same type as the node itself, not just some type conforming to Node. That would be Self in the protocol definition:

protocol Node {
    var parent: Self? { get set }
    var children: [Self] { get set }
}

Now you can define the concrete class (see A Swift protocol requirement that can only be satisfied by using a final class for why the class needs to be final):

final class TreeNode: Node {
    var parent: TreeNode? = nil
    var children: [TreeNode] = []
}

and

func getReversedChildren<T: Node>(node: T) -> [T] {
    return node.children.reversed()
}

compiles without problems.

edited Jan 19, 2019 at 13:34

answered Jan 19, 2019 at 13:26

Martin R

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1 Comment

Oskar Over a year ago

This is it! Thank you :)

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