Demonstration of how using `Enumerator` makes it nicer to work with a data structure that has multiple valid traversals such as a binary tree

tree_enumerator.rb

class Tree
  attr_reader :val, :left, :right

  def self.empty
    EmptyTree.new
  end

  def self.leaf(val)
    new(val, empty, empty)
  end

  def initialize(val, left, right)
    @val = val
    @left = left
    @right = right
  end

  # ==== Traversals ====
  #
  # These methods provide different ways of traversing a tree. See
  # https://en.wikipedia.org/wiki/Tree_traversal#Depth-first_search for more on
  # the different ways a tree can be traversed.
  #
  # The methods can be called with a block and each item will be yielded in that
  # given order:
  #
  #     tree.preorder { |n| puts n }
  #
  # When called without a block they return a `Enumerator`. This allows calling
  # any of the `Enumerable` methods for that given traversal such as:
  #
  #     tree.inorder.map { |n| n * 2 }
  #     tree.postorder.find(&:even?)



  # Depth-first search, pre-order. Given this tree:
  #
  #      1
  #     / \
  #   2     5
  #  / \   / \
  # 3   4 6   7
  #
  # A pre-order traversal will go:
  # 1, 2, 3, 4, 5, 6, 7

  def preorder(&block)
    return to_enum(:preorder) unless block_given?

    block.call(val)
    left.preorder(&block)
    right.preorder(&block)
  end

  # Depth-first search, in-order. Given this tree:
  #
  #      1
  #     / \
  #   2     5
  #  / \   / \
  # 3   4 6   7
  #
  # An in-order traversal will go:
  # 3, 2, 4, 1, 6, 5, 7

  def inorder(&block)
    return to_enum(:inorder) unless block_given?

    left.inorder(&block)
    block.call(val)
    right.inorder(&block)
  end

  # Depth-first search, post-order. Given this tree:
  #
  #      1
  #     / \
  #   2     5
  #  / \   / \
  # 3   4 6   7
  #
  # A post-order traversal will go:
  # 3, 4, 2, 6, 7, 5, 1

  def postorder(&block)
    return to_enum(:postorder) unless block_given?

    left.postorder(&block)
    right.postorder(&block)
    block.call(val)
  end
end

class EmptyTree
  def preorder
  end

  def postorder
  end

  def inorder
  end
end

#      1
#     / \
#   2     5
#  / \   / \
# 3   4 6   7
tree =
  Tree.new(
    1,
    Tree.new(
      2,
      Tree.leaf(3),
      Tree.leaf(4),
    ),
    Tree.new(
      5,
      Tree.leaf(6),
      Tree.leaf(7),
    ),
  )

# (2 * 3) + (12 / 6)
expression =
  Tree.new(
    :+,
    Tree.new(:*, Tree.leaf(2), Tree.leaf(3)),
    Tree.new(:/, Tree.leaf(12), Tree.leaf(6)),
  )