to_enum.rb

class Module
  def to_enum(method_name)
    original_method_name = :"__{method_name}_orig__"
    alias_method original_method_name, method_name
    define_method(method_name) do |*args, &block|
      return enum_for(method_name, *args) unless block
      __send__ original_method_name, *args, &block
    end
  end
end

class A
  def initialize(array)
    @array = array
  end

  to_enum def each_with_plus(n)
    @array.each do |item|
      yield item + n
    end
  end
end

A.new([1,2,3]).each_with_plus(10) do |n|
  p n
end

p(A.new([1,2,3]).each_with_plus(10).map do |n|
  n
end)