output.txt

$ ruby stacked_blocks.rb
START ARRAY THINGS
  START ONE
    START TWO
      START THREE
        START FOUR
          IN THE MIDDLE OF ARRAYS DOING IT
        FINISH FOUR
      FINISH THREE
    FINISH TWO
  FINISH ONE
FINISH ARRAY THINGS

stacked_blocks.rb

class Array
  # Stacks method calls that take blocks
  # so that each method call is nested in the block of the previous one
  def stack_block_calls(callee, &blk)
    empty? ? yield : first.send(callee) { self[1..-1].stack_block_calls(callee, &blk) }
  end
end

$indent = 0;

MSG = ->(msg) {
  ident = " " * $indent
  puts "#{ident}#{msg}"
}

class BlockThing
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def do_it
    $indent += 2
    MSG["START #{name}"]
    yield
  ensure
    MSG["FINISH #{name}"]
    $indent -= 2
  end
end

arry = [
  BlockThing.new("ONE"),
  BlockThing.new("TWO"),
  BlockThing.new("THREE"),
  BlockThing.new("FOUR"),
]

MSG["START ARRAY THINGS"]
arry.stack_block_calls(:do_it) {
  $indent += 2
  MSG["IN THE MIDDLE OF ARRAYS DOING IT"]
  $indent -= 2
}
MSG["FINISH ARRAY THINGS"]