Implement the ruby's flatten method.

flatten.rb

# In Ruby we can easily use flatten method: [[1,2,[3]],4].flatten #=> [1,2,3,4] to do this work.
# I guess it requires me to implement it myself.
module Enumerable
  def my_flatten
    Enumerator.new do |yielder|
      each do |ele|
        if ele.is_a?(Enumerable)
          ele.my_flatten.each do |sub|
            yielder.yield(sub)
          end
        else
          yielder.yield(ele)
        end
      end
    end
  end
end

flatten_spec.rb

require 'minitest/autorun'
require_relative 'flatten'

module Enumerable
  describe "#my_flatten" do

    before do
      @nested_arrays = [[1, 2, [3]], 4]
    end

    it "should flatten the nested arrays" do
      @nested_arrays.flatten.must_equal [1, 2, 3, 4]
    end
  end
end