6.4.2 Enum matching

Enums can be matched by their constructors in a natural way:

var myTree = Node(Leaf("foo"), Node(Leaf("bar"), Leaf("foobar")));
var match = switch (myTree) {
    // matches any Leaf
    case Leaf(_): "0";
    // matches any Node that has r = Leaf
    case Node(_, Leaf(_)): "1";
    // matches any Node that has
    // r = another Node, which has
    // l = Leaf("bar")
    case Node(_, Node(Leaf("bar"), _)): "2";
    // matches anything
    case _: "3";
trace(match); // 2

The pattern matcher will check each case from top to bottom and pick the first one that matches the input value. The following manual interpretation of each case rule helps understanding the process:

  • case Leaf(_): matching fails because myTree is a Node
  • case Node(_, Leaf(_)): matching fails because the right sub-tree of myTree is not a Leaf, but another Node
  • case Node(_, Node(Leaf("bar"), _)): matching succeeds
  • case _: this is not checked here because the previous line matched