Question

I am trying to understand how Enumerator class works. Specifically, I do not know how the yielder object is created and passed to the code block that the constructor takes.

Here is my first try:

class MyEnumerator
  def initialize(&block)
    @block = block
  end 
  def next()
    @block.call self
  end 
  def yield(*args)
    args
  end 
end


num_gen = MyEnumerator.new do |yielder|
  (1..10).each { |num| yielder.yield num }
end

5.times { p num_gen.next }

It is not working, of course because I do not know how to advance the enumerator. Could somebody help me in understanding how I can implement it?

Answer 1

You should use some mechanism of continuation. Check:

http://www.ruby-doc.org/docs/ProgrammingRuby/html/ref_c_continuation.html

http://ruby-doc.org/docs/ProgrammingRuby/html/ref_m_kernel.html#Kernel.callcc

Also, it should be pretty trivial to implement enumerators with fibers (but maybe they are too “high-level” if you want to understand the whole thing, try with continuations then):

http://www.ruby-doc.org/core-1.9.2/Fiber.html