lazy_evaluation.rb

# Basics - if you define an enumerable, you can evaluate it lazily
class Foo
  include Enumerable
  def each(&block)
    (1..20).to_a.each do |i|
      yield i
      puts "yielded #{i}"
    end
  end
end
Foo.new.lazy.take(10).to_a    # => [1,2,3,4,5,6,7,8,9,10]

# Notice that #take will only return the number of elements you allow in #each
class Foo
  include Enumerable
  def each(&block)
    (1..5).to_a.each do |i|
      yield i
      puts "yielded #{i}"
    end
  end
end
Foo.new.lazy.take(10).to_a    # => [1,2,3,4,5]

# So let's do something crazy - we're going to put a non-terminating loop
# inside our #each implementation and see what happens (hint: it's awesome)
class Foo
  include Enumerable
  def each(&block)
    @counter ||= 1
    loop do
      yield @counter
      @counter += 1
    end
  end
end
Foo.new.lazy.take(10).to_a    # => [1,2,3,4,5,6,7,8,9,10]

# And we can still terminate one some condition
class Foo
  include Enumerable
  def each(&block)
    @counter ||= 1
    while @counter <= 5 do
      yield @counter
      @counter += 1
    end
  end
end
Foo.new.lazy.take(10).to_a    # => [1,2,3,4,5]


# Ruby is awesome; QED