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

It seems to be useful when you have multiple objects you want to enumerate over, but flat_map isn't suitable, and you want to chain the enumeration with another action:

module Enumerable
  def count_by
    items_grouped_by_criteria = group_by {|object| yield object}
    counts = items_grouped_by_criteria.map{|key, array| [key, array.length]}
    Hash[counts]
  end
end

def calculate_letter_frequencies
  each_letter.count_by {|letter| letter}
end

def each_letter
  filenames = ["doc/Quickstart", "doc/Coding style"]
  # Joining the text of each file into a single string would be memory-intensive
  enumerator = Enumerator.new do |yielder|
    filenames.each do |filename|
      text = File.read(filename)
      text.chars.each {|letter| yielder.yield(letter)}
    end
  end
  enumerator
end

calculate_letter_frequencies