fp_style.rb

require 'prettyprint' # for the p function

module Enumerable
  alias :fold :inject
end

root = [["root", [
                  ["child", [
                             ["grandkid", []],
                             ["grandkid2", []]]],
                  ["child2", []]]]]

thunk = lambda do |a, (name, children)|
  a << name.reverse
  children.fold(a, &thunk)
end

p root.fold([], &thunk)

fp_style_2.rb

require 'prettyprint'

# This form of the FP style creates a definition for fold()
# for trees of the form [<value>, [<children>]]
module FoldableTree

  # define fold for trees of the form [<value>, [<children>]]
  # recursively applies the FoldableTree type to each node
  def fold accumulator, &f
    # note the use of structural decomposition,
    # as (value, children) for elements in the tree
    inject(accumulator) do |accumulator, (value, children)|
      a2 = f.call accumulator, value
      children.extend FoldableTree
      children.fold a2, &f
    end
  end
end

root = [["root", [
                  ["child", [
                             ["grandkid", []],
                             ["grandkid2", []]]],
                  ["child2", []]]]]

# Declare our tree to be an instance of FoldableTree
root.extend FoldableTree

# Fold across our foldable tree, collecting names in reverse
p root.fold([]) do |a, name|
  # create a new array each time, rather than mutate a
  [a, name.reverse].flatten
end

oo_style.rb

require 'prettyprint' # for the p function

class Tree
  attr_accessor :name

  def initialize name
    @name = name
    @children = []
  end

  def visit visitor
    visitor.visit(self)
    for child in @children
      child.visit(visitor)
    end
    visitor.value
  end

  def << child
    @children << child
    self
  end
end

root = Tree.new("root") << (Tree.new("child") << Tree.new("grandkid") << Tree.new("grandkid2")) << Tree.new("child2")

class Visitor
  attr_accessor :value

  def initialize
    @value = []
  end

  def visit node
    @value << node.name.reverse
  end
end

p root.visit(Visitor.new)