Implementation of Enumerable.map() in Ruby

mappable.rb

# Run using rspec.
#   e.g.: rspec file_name.rb

require 'rubygems'
require 'rspec'

module Enumerable
  def mymap (&block)
    return nil if block.nil?

    self.each_with_index do |e, i|
      self[i] = block.call(e)
    end
  end
  alias :collect :map
end

describe Enumerable do
  describe '#mymap' do
    let(:arr) { Array(1..5) }

    it 'should return nil if no arguments are used' do
      arr.mymap.should == nil
    end

    it 'should return itself if the block does nothing' do
      arr.mymap{ |e| e }.should == [1, 2, 3, 4, 5]
    end

    it 'should return +1 to each element' do
      arr.mymap{ |e| e+1 }.should == [2, 3, 4, 5, 6]
    end
  end
end