Then

Kotlin has scope functions¹ — functions that let you run a block of code in the context of an object. Swift has no such mechanism, but you can either build one yourself or use a ready-made one.

How does it work in Kotlin? Let’s look at a simple example. Suppose we have a tree that looks like this:

class TreeNode(var `val`: Int) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

And we’d like to declare the entire tree we need “in one go”. The apply() scope function will help us do that:

TreeNode(1).apply {
    left = TreeNode(2)
    right = TreeNode(3)
}

What do we need to bring similar behavior to Swift? It’s not hard, really. We can create an extension with the following method:

public func then(_ block: (Self) throws -> Void) rethrows -> Self {
    try block(self)
    return self
}

This snippet will be used as follows:

public class TreeNode {
    public var val: Int
    public var left: TreeNode?
    public var right: TreeNode?

    init(val: Int) {
        self.val = val
    }
}

extension TreeNode {
    public func then(_ block: (Self) throws -> Void) rethrows -> Self {
        try block(self)
        return self
    }
}

Now we can assemble the tree (or any more complex object) by describing its initialization just once:

let tree = TreeNode(val: 1).then { it in
    it.left = TreeNode(val: 2).then { it in
        it.left = TreeNode(val: 5)
        it.right = TreeNode(val: 7)
    }
    it.right = TreeNode(val: 3)
}

But in fact, someone has already thought of this and done all the work for us. All the sugar described above is handled nicely by the Then² library. It’s installed as easily as any other dependency:

...
    dependencies: [
        .package(url: "https://github.com/devxoul/Then", exact: "3.0.0"),
    ],
...

Don’t forget to add the dependency itself to your build target:

...
        .target(
            name: "Solution",
            dependencies: ["Then"]
        ),
...

If you’re working with something other than the built-in types, add a Then extension to your own type:

extension ListNode: Then {}