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

The core libraries insert a guard return to_enum(:name_of_this_method, arg1, arg2, ..., argn) unless block_given?. In your case:

class Array
  def double
    return to_enum(:double) unless block_given?
    each { |x| yield 2*x }
  end
end

>> [1, 2, 3].double { |x| puts(x) }
2
4
6 
>> ys = [1, 2, 3].double.select { |x| x > 3 } 
#=> [4, 6]