Mutating a Node of a general tree in Scala - scala

Question

Consider the following standard class:
class Node(val data: NodeData, val children: Seq[Node])
The NodeData class is also simple:
case class NodeData(text: String, foo: List[Bar])
Also, the tree has arbitrary depth, it's not fixed.
Clearly, implementing a breath-first or depth-first search on that structure is trivial with idiomatic Scala. However, consider that I want not only to visit each of these nodes, but I also want to mutate them on each visit. More concretely, I want to mutate an object in that foo list. How would I go about implementing this? One way I thought about this is to somehow update the nodes and build a new tree while traversing it, but my intuition tells me there is a simpler solution than that.

Answer 1

If you really want to stay immutable, I would define a function recMap on Node, like this:
def recMap(f: NodeData => NodeData) : Node = Node(f(data), children.map(_.recMap(f)))
You could then use it like in this example (I made Node a case class too):
type Bar = Int
case class NodeData(text: String, foo: List[Bar])
case class Node(data: NodeData, children: Seq[Node]) {
def recMap(f: NodeData => NodeData) : Node = Node(f(data), children.map(_.recMap(f)))
}
val tree = new Node(NodeData("parent", List(1, 2, 3, 4)), Seq(
Node(NodeData("a child", List(5, 6, 7, 8)), Seq.empty),
Node(NodeData("another child", List(9, 10, 11, 12)), Seq.empty)
))
val modifiedTree = tree.recMap(
data => NodeData(
if(data.text == "parent") "I am the parent!" else "I am a child!",
data.foo.filter(_ % 2 == 0)
)
)
println(modifiedTree)
Try it out!
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