From: Rudolf Polzer <abuse@...>
Date: 2003-04-03T07:53:21+09:00
Subject: Generators again

I didn't understand the Generators using callcc() example I read some
time ago, so I tried it on my own. Maybe my class is more understandable
to others, too...

Somehow my approach reminded me of good old DOS times (cooperative
multitasking implemented in Turbo Pascal)... and in fact, cooperative
multitasking could be implemented by round-robin calling generators.
The usage of that 'no' variable might remind of C a bit... (I just
say setjmp/longjmp), although this thing doesn't seem possible with C:

   The stack context will be invalidated  if  the  function  which
   called setjmp() returns.

(and that's exactly what those TP hacks did: they used a seperate stack
 segment for every cooperative "thread")

I think this is the main difference between continuations and
setjmp/longjmp.


class Generator
  def initialize(&block)
    pr = lambda() do |r|
      r[0]
      block[r]
    end
    no = 0
    callcc() do |cont| @cont = cont end
    no += 1
    if no == 1 then
      pr[lambda() do |v|
        @retval = v
        get_out_of_here()
      end]
      lastcont = @cont
      @cont = nil
      lastcont.call()
    end
  end
  def next!()
    raise "read beyond end of generator" if @cont == nil
    get_out_of_here()
    return nil if @cont == nil
    return @retval
  end
  def get_out_of_here()
    no = 0
    lastcont = @cont
    callcc() do |cont| @cont = cont end
    no += 1
    if no == 1 then
      lastcont.call()
    end
  end
  private :get_out_of_here
end

# that's a generator using another one
def summation(gen)
  Generator.new() do |send|
    sum = 0
    while v = gen.next!()
      sum += v
      send[sum]
    end
  end
end

# a sample generator
fib = Generator.new() do |send|
  a, b = 0, 1
  while true
    a, b = b, a+b
    send[a]
  end
end

fibsum = summation(fib)

while true
  p fibsum.next!()
end