Use `self` as a default parameter

Question

Can I use self as a default parameter in Swift? This code feels pretty straight forward but I don't understand the message the compiler is kicking back:

func printTree(node: TreeNode = self, tabs: String = "") {   println(tabs + node.name!)   node.children.forEach { printTree(node: $0, tabs: tabs+"\t") } } 

Error:

'TreeNode -> () -> TreeNode' is not convertible to 'TreeNode' 

Huh?

There are probably other ways I could solve the tree traversal but I'm actually just curious about the default parameter restriction. Is this a real thing? Is there a mention of this in the docs? I couldn't find one.

Update:

I did this from scratch with no dependencies (I had a class heirarchy and a custom forEach monkey patch involved before). It still errors:

class PeanutButterJelly {   var children: [PeanutButterJelly]?   func doDance(){ println("dancing") }   func everybodyDanceNow(pbj: PeanutButterJelly = self) {     pbj.doDance()     if let children = pbj.children {       for child in children { child.doDance() }     }   } } 

Error:

Swift compilation error: unresolved identifier 'self' 

Xcode 6.3.2

Answer 1

Can I use self as a default parameter in Swift

No. This is basically for the same reason that you can't say self when initializing an instance method:

class TreeNode {     var otherNode : TreeNode = self // Use of unresolved identifier `self`     func printTree(tree:TreeNode = self) { } // Use of unresolved identifier `self` } 

At the time you're defining this entity (property or method), there is no such thing as self. There will be a self later, when this method is called, inside the body of the function - because the function will be called on some instance, namely self.

One easy way to do what you want to do is to make this parameter an Optional with a nil default value. Then, in the body of the function, check for nil and use self:

class TreeNode {     func printTree(tree:TreeNode? = nil) {         let treeToPrint = tree ?? self         // do stuff     } } 

Now it's legal to call printTree with no parameter, and it does what you want it to do:

TreeNode().printTree()