Ever want to combine two or more Enumerables (or anything that responds to `#to_enum`) into a single one, without reading them first? Ruby 1.9+'s Enumerator is pretty badass. Behold!

a_poor_devs_cat.rb

def poor_devs_cat *args
  args = args.to_enum

  Enumerator.new do |yielder|
    enum = args.next.to_enum

    loop do
      begin
        yielder << enum.next
      rescue StopIteration
        enum = args.next.to_enum
      end
    end
  end
end

cat = poor_devs_cat File.open("part1.txt"), File.open("part2.txt"), File.open("part3.txt")

cat.each_with_index do |line,idx|
  puts "#{idx+1}: #{line.upcase}"
end

output.txt

1: BICYCLE BICYCLE BICYCLE
2: I WANT TO RIDE MY BICYCLE BICYCLE BICYCLE
3: I WANT TO RIDE MY BICYCLE
4: I WANT TO RIDE MY BIKE
5: I WANT TO RIDE MY BICYCLE
6: I WANT TO RIDE IT WHERE I LIKE
7: YOU SAY BLACK, I SAY WHITE
8: YOU SAY BARK, I SAY BITE
9: YOU SAY SHARK, I SAY HEY MAN
10: JAWS WAS NEVER MY SCENE
11: AND I DON'T LIKE STAR WARS
12: YOU SAY ROLLS I SAY ROYCE
13: YOU SAY GOD GIVE ME A CHOICE
14: YOU SAY LORD I SAY CHRIST
15: I DON'T BELIEVE IN PETER PAN
16: FRANKENSTEIN OR SUPERMAN
17: ALL I WANNA DO IS

part1.txt

Bicycle bicycle bicycle
I want to ride my bicycle bicycle bicycle
I want to ride my bicycle
I want to ride my bike
I want to ride my bicycle
I want to ride it where I like

part2.txt

You say black, I say white
You say bark, I say bite
You say shark, I say hey man
Jaws was never my scene
And I don't like Star Wars

part3.txt

You say Rolls I say Royce
You say God give me a choice
You say Lord I say Christ
I don't believe in Peter Pan
Frankenstein or Superman
All I wanna do is