ruby 1.8.7 Enumerator

do_stuff.rb

def do_stuff(enumerator)
  # stuff
end
do_stuff([1,3,5].reverse_each)
do_stuff([1,3,5].each)

each_char.rb

"rails".each_char.sort

each_with_index.rb

%w(fun with enumerator).each_with_index.map { |value, i| "#{i+1}. #{value}" }

enum_for.rb

%w(fun with enumerator).enum_for(:each_with_index).map { |value, i| "#{i+1}. #{value}" }

extend.rb

class Array
  [:find, :find_all, :partition, :reject, :select, :sort_by].each do |method|
    class_eval <<-EOV
      def #{method}_with_enumerator(*args)
        if block_given?
          #{method}_without_enumerator(*args)
        else
          Enumerable::Enumerator.new(self, :enumerator, *args)
        end
      end
    EOV
    alias_method_chain method
  end
end

factorial.rb

n.downto(1).inject(:*)