does this

typealias Visitor<T> = (TreeNode<T>) -> Unit

mean, “any function that takes TreeNode<T> and return Unit can now be referred to as

Visitor<T>

? so instead of writing

fun search(printResult: (TreeNode<T>) -> Unit)

I write

fun search(visit: Visitor<T>)

Yes and the compiler will replace the Visitor<T> with the correct type. It doesn’t create a new type just a simpler short hand for your code. https://kotlinlang.org/docs/type-aliases.html