From: Shin-ichiro HARA <sinara@...>
Date: 2003-03-18T14:21:08+09:00
Subject: Re: Iterator using Continuations

Paul wrote:
>This looks like a reasonable approach, particularly for an example.

I think so too, and add more comments:

  1) If the enumerable object has nil, hasNext returns false.
    It will stop the loop.
       ex) iter = Iterator.new([1, nil, 5])
           while iter.hasNext? #=> stops at nil
             puts iter.next
           end
  2) The symmetry of @back and @next is ignored.

The following is a little improvement of the original code:

class Iterator
  def initialize(enum)
    @next = nil
    @back = nil

    enum.each do |el|
      @current = el
      callcc do |cc|
        @next = cc
        if @back then @back.call else return end
      end
    end
    @next = nil
    @back.call if @back
  end

  def hasNext?
    @next
  end

  def next
    last = @current
    callcc do |cc|
      @back = cc
      @next.call if @next
    end
    last
  end
end


As others said, the library using callcc is not a tool for real
applications. But it attracts theoritical interest. I collected
the very short codes for Continuation:

  http://blade.nagaokaut.ac.jp/~sinara/ruby/callcc-lib/

It includes the generic library CoThread. Using this, the iterator
can be written as follows:

require "co-thread"
class Iterator
  def initialize(enum)
    @ct = CoThread.new
    @ct.fork do
      prev = nil
      @more = true
      enum.each do |x|
	@ct.pass(prev)
	prev = x
      end
      @more = false
      @ct.pass(prev)
    end
    @ct.pass
  end

  def hasNext?
    @more
  end

  def next
    @ct.pass
  end
end


Shin-ichiro HARA