my_flatten.rb

require 'minitest/autorun'

class TestMyFlatten < Minitest::Test
  def test_deeply_nested_arrays
    assert_equal [1,2,3,4], MyFlatten.new([[1,2,[3]],4]).flatten
  end

  def test_argument_error
    assert_raises ArgumentError do
      MyFlatten.new("not an array")
    end
  end
end

class MyFlatten
  def initialize(original)
    raise ArgumentError unless original.is_a?(Array)
    @original = original
  end

  def flatten
    flattened = []
    original.each do |element|
      if element.is_a?(Array)
        flattened += self.class.new(element).flatten
      else
        flattened << element
      end
    end
    flattened
  end

  private

  attr_reader :original
end