How to create your own Ruby Enumerator

enumerator.rb

# Enumerator is a very powerful feature if you know how to use it. The most
#   powerful part of Enumerator is the possibility of chaining them. Take some
#   time to understand the intricacies of it because it is not very well
#   documented. Some awesome references that I found are listed below. Note that
#   I didn't understand when reading any of them right off the bat. Read a few,
#   dig some documentation and experiment for yourself to get the "fuller"
#   picture.
#
#   [1]: https://blog.arkency.com/2014/01/ruby-to-enum-for-enumerator/
#   [2]: https://stackoverflow.com/a/9194052
#   [3]: https://ruby-doc.org/core-2.7.1/Enumerator.html

class MyClass
  def an_iterator
    # enum_for and to_enum is the same. [1]
    return enum_for(:an_iterator) if not block_given?

    # Use of yield here is of utmost importance, any code from here on will be
    #   executed by the enumerator over and over again.
    index = 0
    while index <= 10
      yield(index += 2)
    end
  end

  def an_iterator_but_doesnt_have_yield
    return enum_for(:an_iterator_but_doesnt_have_yield) if not block_given?
  end
end
my_class = MyClass.new


################
#### Example of using an enumerator
################

## Without block
iterator1 = my_class.an_iterator # returns you an iterator.
puts iterator1.take(5)
# => [2, 4, 6, 8, 10]
puts iterator1.next
# => 12

## With block
iterator2 = my_class.an_iterator # returns you an iterator.
iterator2.each do |x|
  # #each returns another enumerator enumerating #an_iterator method. x is what
  #   is yielded when calling #next on iterator 2.
  puts x
end
# 2
# 4
# 6
# 8
# 10
# => nil


################
#### Example of using an iterator that does not have yield
################

## Without block
missing_yield_iterator1 = my_class.an_iterator_but_doesnt_have_yield
puts missing_yield_iterator1.take(10)
# []
puts missing_yield_iterator1.next
# StopIteration: iteration reached an end

## With block
missing_yield_iterator2 = my_class.an_iterator_but_doesnt_have_yield
missing_yield_iterator2.each do |x|
  puts x
end
# => nil