When is the Enumerator::Yielder#yield method useful?

Question

The question \"Meaning of the word yield\" mentions the Enumerator::Yielder#yield method. I haven\'t used it before, and wonder under what circumstances it would be

Answer 1

"How to create an infinite enumerable of Times?" talks about constructing and lazy iterators, but my favorite usage is wrapping an existing Enumerable with additional functionality (any enumerable, without needing to know what it really is, whether it's infinite or not etc).

A trivial example would be implementing the each_with_index method (or, more generally, with_index method):

module Enumerable
  def my_with_index
    Enumerator.new do |yielder|
      i = 0
      self.each do |e|
        yielder.yield e, i
        i += 1
      end
    end
  end

  def my_each_with_index
    self.my_with_index.each do |e, i|
      yield e, i
    end
  end
end

[:foo, :bar, :baz].my_each_with_index do |e,i|
  puts "#{i}: #{e}"
end
#=>0: foo
#=>1: bar
#=>2: baz

Extending to something not already implemented in the core library, such as cyclically assigning value from a given array to each enumerable element (say, for coloring table rows):

module Enumerable
  def with_cycle values
    Enumerator.new do |yielder|
      self.each do |e|
        v = values.shift
        yielder.yield e, v
        values.push v
      end
    end
  end
end

p (1..10).with_cycle([:red, :green, :blue]).to_a # works with any Enumerable, such as Range
#=>[[1, :red], [2, :green], [3, :blue], [4, :red], [5, :green], [6, :blue], [7, :red], [8, :green], [9, :blue], [10, :red]]

The whole point is that these methods return an Enumerator, which you then combine with the usual Enumerable methods, such as select, map, inject etc.