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Change node in Scala case class tree
abstract class Tree
case class Branch(b1:Tree,b2:Tree, value:Int) extends Tree
case class Leaf(value:Int) extends Tree
var tree = Branch(Branch(Leaf(1),Leaf(2),3),Branch(Leaf(4), Leaf(5),6))

// Changed Tree to Node, b/c that seems more accurate
// Since Branch and Leaf both have value, pull that up to base class
sealed abstract class Node(val value: Int) {
  /** Replaces this node or its children according to the given function */
  def replace(fn: Node => Node): Node

  /** Helper to replace nodes that have a given value */
  def replace(value: Int, node: Node): Node =
    replace(n => if (n.value == value) node else n)
}

// putting value first makes class structure match tree structure
case class Branch(override val value: Int, left: Node, right: Node)
     extends Node(value) {
  def replace(fn: Node => Node): Node = {
    val newSelf = fn(this)

    if (this eq newSelf) {
      // this node's value didn't change, check the children
      val newLeft = left.replace(fn)
      val newRight = right.replace(fn)

      if ((left eq newLeft) && (right eq newRight)) {
        // neither this node nor children changed
        this
      } else {
        // change the children of this node
        copy(left = newLeft, right = newRight)
      }
    } else {
      // change this node
      newSelf
    }
  }
}

val tree1 = Branch(Branch(Leaf(1),Leaf(2),3),Branch(Leaf(4), Leaf(5),6))
val tree2 = tree1.copy(b2 = tree1.b2.copy(b1 = Leaf(5))
// -> Branch(Branch(Leaf(1),Leaf(2),3),Branch(Leaf(5), Leaf(5),6))