pull.rb

require 'psych'
require 'thread'

class PullParser
  def initialize io
    @queue = SizedQueue.new 10
    parser = Psych::Parser.new self
    @thread = Thread.new {
      begin
        parser.parse io
      ensure
        @queue.push nil
      end
    }
  end

  def next_token
    @queue.pop
  end

  def finish
    @thread.join
  end

  private
  def method_missing name, *args
    @queue << [name, args]
  end
end

pp = PullParser.new DATA
while token = pp.next_token
  p token
  # do something
end

pp.finish

__END__
---
- foo
- foo
- foo
- foo
- foo
- foo
- foo
- foo