Kotlin function parameter: Val cannot be reassigned

Question

I have written Red–black tree in Kotlin. Fun insertFixup restores balance after inserting new element (z: Node? is new element). Algorithm of tree balancin

Answer 1

Kotlin function parameters are read-only values and are not assignable.

You can however create a ReadWriteProperty object to pass to insertFixup for getting/setting newNode:

...
class BinarySearchTree {
...
    fun insertFixup(zProperty: ReadWriteProperty) {
        var z by zProperty
...

fun main(args: Array) {
    val bst = BinarySearchTree()

    var newNode: Node? = null
    val newNodeProperty = object : ReadWriteProperty {
        override operator fun getValue(thisRef: Any?, property: KProperty<*>): Node? {
            return newNode
        }

        override operator fun setValue(thisRef: Any?, property: KProperty<*>,
                                       value: Node?) {
            newNode = value
        }
    }

    while (true) {
        newNode = Node(readLine()!!.toInt())
        bst.insert(newNode!!)
        bst.insertFixup(newNodeProperty)
    }
}

And if you are willing to use a property instead of a variable then you can use a property reference for getting/setting newNode from insertFixup instead:

...
class BinarySearchTree {
...
    fun insertFixup(zProperty: KMutableProperty0) {
        var z by zProperty
...

var newNode: Node? = null

fun main(args: Array) {
    val bst = BinarySearchTree()

    while (true) {
        newNode = Node(readLine()!!.toInt())
        bst.insert(newNode!!)
        bst.insertFixup(::newNode)
    }
}

// the following allow `KMutableProperty0` to be used as a read/write delegate
operator fun  KProperty0.getValue(thisRef: Any?, property: KProperty<*>): T = get()
operator fun  KMutableProperty0.setValue(thisRef: Any?, property: KProperty<*>, 
                                               value: T) = set(value)