Solution to the coin changer kata

gistfile1.txt

class Object
  def when_defined(primary, alternate)
    primary.call(self)
  end
end

class NilClass
  def when_defined(primary, alternate)
    alternate.call(self)
  end
end

module Bank
  module_function

  COINS = [25, 10, 5, 1]

  def change(amount)
    COINS.
      find { |coin| amount >= coin }.
      when_defined ->(coin) { [coin] + change(amount - coin) },
                   ->(coin) { [] }
  end
end