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