ruby methods that either yield or return Enumerator

Question

in recent versions of Ruby, many methods in Enumerable return an Enumerator when they are called without a block:

[1,2,3,4].map 
#=>         


        

Answer 1

use Enumerator#new:

class Array
  def double(&block)
    Enumerator.new do |y| 
      each do |x| 
        y.yield x*2 
      end 
    end.each(&block)
  end
end