array_ext.rb

class Array
  # array.to_flatten -> new array
  #
  # Returns a new array that is a one-dimensional flattening of +self+
  # (recursively).
  #
  # That is, for every element that is an array, extract its elements into
  # the new array.
  #
  #    a = [[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10]
  #    a.to_flatten #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  def to_flatten
    self.inject([]) do |result, value|
      if value.is_a?(Array)
        result += value.to_flatten
      else
        result << value
      end
    end
  end
end

array_spec.rb

require 'spec_helper'

describe Array do
  describe '#to_flatten' do
    subject { [[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10] }

    it 'returns a new array that is a one-dimensional flattening of self' do
      expect(subject.to_flatten).to eql([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
    end

    context 'with flatten array' do
      subject { [1, 2, 3] }

      it 'returns a new array' do
        expect(subject.to_flatten).not_to be subject
        expect(subject.to_flatten).to eql subject
      end
    end

    context 'with empty array' do
      subject { [] }

      it 'returns a new empty array' do
        expect(subject.to_flatten).to be_empty
      end
    end
  end
end