[Stackless] Video on channels

[Grant Olson]

I finally got around to watching a techtalk Rob Pike did on the langauage
Newsqueak, which uses channels extensively.  It's obviously not stackless
specific, but gives plenty of good motivations for channels as the way of
the future.

http://video.google.com/videoplay?docid=810232012617965344

In particular, there was a really elegant prime number sieve.  You really
need to see the slide explaining it to see how brilliant it is.  Anyway,
here's the stackless version of the sieve:

from stackless import *

class sieve:
    def __init__(self, start_num):
        self.ch = channel()
        self.start_num = start_num
        self.next_sieve = None
        print start_num
        tasklet(self.listen)()


    def listen(self):
        while 1:
            current_num = self.ch.receive()
            if current_num % self.start_num == 0:
                pass # not prime
            else:
                if not self.next_sieve:
                    self.next_sieve = sieve(current_num)
                self.next_sieve.ch.send(current_num)

class counter:
    def __init__(self,number,send_to):
        self.number = number
        self.ch = channel()
        self.send_to = send_to
        tasklet(self.listen)()

    def listen(self):
        while 1:
            self.ch.receive()
            self.send_to.send(self.number)
            self.number += 1
    
    def send(self):
        self.ch.send(1)


prime_sieve = sieve(2)
current_counter = counter(2,prime_sieve.ch)

for i in range(100):
    current_counter.send()