From: Ross Bamford <rossrt@...>
Date: 2006-03-24T22:49:41+09:00
Subject: Re: iterator class not working

On Fri, 2006-03-24 at 21:38 +0900, Peter Ertl wrote:
> Hi,
> 
> in order to understand ruby better (and for fun reasons of course) I try to
> write an iterator class. this way I want to understand  'binding' and
> 'callcc' better.

Here is a slightly modified version of your code that doesn't cover
everything, but should get you going in the right direction.

##### code #####
class Iterator
  def initialize(enum)
    @yield = lambda do
      enum.each do |item|
        @yield = callcc { |cc|
          @next.call cc, item
        }
      end
      raise "Exhausted"
    end
  end

  def next
    @yield, item = callcc do |cc|
      @next = cc
      @yield.call
    end
    item
  end
end

names = %w{mary gordy john jane elwood}

it = Iterator.new(names)

6.times do
    puts it.next
end

##### outputs #####

mary
gordy
john
jane
elwood
-:9:in `initialize': Exhausted (RuntimeError)
        from -:16:in `next'
        from -:14:in `next'
        from -:27
        from -:26

All you need to bear in mind when using continuations this way is that
what you're effectively needing is to 'emulate' two call stacks on a
single thread (maybe your 'next' stack and your 'yield' stack) and to
chop between them by calling the appropriate continuation.

In some respects it's similar to having two threads in a synchronized
stop/start setup, which is another way you can implement the above. In
fact...

	http://www.rubyquiz.com/quiz66.html

I'll leave the details (proper end handling and so on) to you. Btw, I
know this doesn't use binding but I found that to be a fairly
unintuitive way to do it. The binding use in your original code was fine
in itself I think, but you can't use it to retain state the way I think
you were hoping for.

-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk