Conforming a new protocol to Sequence with a default makeIterator() implementation - swift

Question

I made a (very basic) BinaryTree protocol:
public enum BinaryTreeChildSide {
case left, right
}
public protocol BinaryTree {
associatedtype Element
associatedtype Index
func child(of index: Index, side: BinaryTreeChildSide) -> Index?
var rootIndex: Index? { get }
subscript(position: Index) -> Element { get }
}
For a basic iterative in-order traversal, I made a BinaryTreeIterator (note that I don't implement Sequence just yet):
public extension BinaryTree {
func makeIterator() -> BinaryTreeIterator<Self> {
return BinaryTreeIterator(self)
}
}
public struct BinaryTreeIterator<Tree: BinaryTree>: IteratorProtocol {
private let tree: Tree
private var stack: [Tree.Index]
private var index: Tree.Index?
private init(_ tree: Tree) {
self.tree = tree
stack = []
index = tree.rootIndex
}
public mutating func next() -> Tree.Element? {
while let theIndex = index {
stack.append(theIndex)
index = tree.child(of: theIndex, side: .left)
}
guard let currentIndex = stack.popLast() else { return nil }
defer { index = tree.child(of: currentIndex, side: .right) }
return tree[currentIndex]
}
}
Implementing a binary heap to this protocol is also pretty straight-forward:
public struct BinaryHeap<Element> {
private var elements: [Element]
public init(_ elements: [Element]) {
self.elements = elements
}
}
extension BinaryHeap: BinaryTree {
private func safeIndexOrNil(_ index: Int) -> Int? {
return elements.indices.contains(index) ? index : nil
}
public func child(of index: Int, side: BinaryTreeChildSide) -> Int? {
switch side {
case .left: return safeIndexOrNil(index * 2 + 1)
case .right: return safeIndexOrNil(index * 2 + 2)
}
}
public var rootIndex: Int? { return safeIndexOrNil(0) }
public subscript(position: Int) -> Element {
return elements[position]
}
}
So far, so good. I can now make a simple heap and iterate through its elements:
let heap = BinaryHeap([4, 2, 6, 1, 3, 5, 7])
var iterator = heap.makeIterator()
while let next = iterator.next() {
print(next, terminator: " ")
}
// 1 2 3 4 5 6 7
This works, but of course the goal of implementing makeIterator() is to conform to Sequence. however, if I replace
public protocol BinaryTree {
with
public protocol BinaryTree: Sequence {
then the compiler complains that BinaryHeap doesn't implement Sequence because the associated type Iterator couldn't be inferred. If I manually specify the Iterator type with
extension BinaryHeap: BinaryTree {
typealias Iterator = BinaryTreeIterator<BinaryHeap>
...
}
then the compiler shows an error that Iterator circularly references itself. So that might be why the Iterator type couldn't be inferred.
Interestingly, it works if I wrap my custom BinaryTreeIterator in an AnyIterator instance:
public extension BinaryTree {
func makeIterator() -> AnyIterator<Element> {
return AnyIterator(BinaryTreeIterator(self))
}
}
let heap = BinaryHeap([4, 2, 6, 1, 3, 5, 7])
for number in heap {
print(number, terminator: " ")
}
// 1 2 3 4 5 6 7
Apple's own IndexingIterator seems to work in a similar fashion to my BinaryTreeIterator:
public struct IndexingIterator<
Elements : IndexableBase
// FIXME(compiler limitation):
// Elements : Collection
> : IteratorProtocol, Sequence {
...
}
From the source code. Maybe the problem I'm facing might also be because of the compiler limitation mentioned there, but I don't know for sure.
Is there a way to conform BinaryTree to Sequence without using AnyIterator?

Answer 1

This is the farthest I could take it. Now the compiler will still complain about the heap not containing any makeIterator member
(which I thought was included by default once someone conforms to sequence—wrong turns out one must conform to Sequence ​and​ IteratorProtocol for the default implementation) and having a next—but once you add those methods its smooth sailing.
So makeIterator + next method to make Mr/Mrs/Preferred Gender Pronoun compiler happy.
public enum BinaryTreeChildSide {
case left, right
}
public struct BinaryTreeIterator<Tree: BinaryTree>: Sequence, IteratorProtocol {
private let tree: Tree
private var stack: [Tree.Index]
private var index: Tree.Index?
private init(_ tree: Tree) {
self.tree = tree
stack = []
index = tree.rootIndex
}
public mutating func next() -> Tree.Element? {
while let theIndex = index {
stack.append(theIndex)
index = tree.child(of: theIndex, side: .left)
}
guard let currentIndex = stack.popLast() else { return nil }
defer { index = tree.child(of: currentIndex, side: .right) }
return tree[currentIndex]
}
}
public protocol BinaryTree: Sequence {
associatedtype Element
associatedtype Index
func child(of index: Index, side: BinaryTreeChildSide) -> Index?
var rootIndex: Index? { get }
subscript(position: Index) -> Element { get }
}
extension BinaryTree {
func makeIterator() -> BinaryTreeIterator<Self> {
return BinaryTreeIterator(self)
}
}
extension BinaryHeap {
private func safeIndexOrNil(_ index: Int) -> Int? {
return elements.indices.contains(index) ? index : nil
}
public func child(of index: Int, side: BinaryTreeChildSide) -> Int? {
switch side {
case .left: return safeIndexOrNil(index * 2 + 1)
case .right: return safeIndexOrNil(index * 2 + 2)
}
}
public var rootIndex: Int? { return safeIndexOrNil(0) }
public subscript(position: Int) -> Element {
return elements[position]
}
}
public struct BinaryHeap<Element> {
private var elements: [Element]
public init(_ elements: [Element]) {
self.elements = elements
}
}
let heap = BinaryHeap([4, 2, 6, 1, 3, 5, 7])
var iterator = heap.makeIterator()
while let next = iterator.next() {
print(next, terminator: " ")
}

Answer 2

Apparently it was a Swift bug: my code compiles fine using Swift 3.1.