gistfile1.rbx

class TreeBuilder
  def build a
    root_node = Node.new(a)
    yield root_node if block_given?
    root_node.as_tree
  end
  
  class Node
    def initialize value
      @value = value
      @children = []
    end
    
    def as_tree
      (@children.empty?) ? @value : [@value, @children.map(&:as_tree)]
    end
    
    def child c
      new_node = Node.new(c)
      @children << new_node
      yield new_node if block_given?
    end
  end
end

builder = TreeBuilder.new

results = builder.build(1) do |root|
  root.child(2)
  root.child(3) do |three|
    three.child(4)
  end
end
p results
# <= [1, [2, [3, [4]]]]

class TreeWalker
  def walk tree, &combine_func
    root, children = *tree
    inner_walk root, children, &combine_func
  end

  private
  def inner_walk root, children, &combine_func
    children ||= []
    combined_children = children.map { |c|
      r, cc = *c
      inner_walk combine_func.call(root, r), cc, &combine_func
    }
    [root] + combined_children
  end
end

p TreeWalker.new.walk(results) { |p, c| [p,c].join(',') }
# <= [1, ["1,2"], ["1,3"], ["1,3,4"]]]