Re: searching an explanation: call/cc and 'do' strangeness

[roland.kaufmann@space.at]

>>>>> "P" == Pierpaolo Bernardi <bernardp@cli.di.unipi.it> wrote:

    P> On 27 Apr 1999, Gary Houston wrote:

    >> Hmm, folk-law says that continuations can be used to simulate
    >> threads, but I've never seen it done.

    P> Once upon a time, Siskind posted an implementation of threads
    P> built on call/cc on the scheme mailing list.

    P> P.

I guess you mean this one:

  Subject: Re: Threads and call/cc
  Newsgroups: comp.lang.scheme
  Date: Thu, 11 Feb 1999 23:47:39 GMT

  Here is some sample code that will get you started.  Its very minimal
  (only 18 lines of Scheme for the thread primitives) but it serves to
  get the basic idea of how to do threads with call/cc.  Once you
  understand the basic idea you can improve on the scheduling algorithm
  and thread queue data structure.

  Marc Feeley


  (define thread-queue '())

  (define (add-thread proc)
    (set! thread-queue (append thread-queue (list proc))))

  (define (fork-thread thunk)
    (add-thread (lambda (dummy) (thunk) (terminate-thread))))

  (define (switch-thread)
    (call-with-current-continuation
      (lambda (cont) (add-thread cont) (terminate-thread))))

  (define (terminate-thread)
    (if (null? thread-queue)
      (exit)
      (let ((cont (car thread-queue)))
	(set! thread-queue (cdr thread-queue))
	(cont 'dummy))))

  ; A test of the thread routines:

  (define (repeat x n)
    (if (> n 0)
      (begin (switch-thread) (write x) (repeat x (- n 1)))))

  (fork-thread (lambda () (repeat 'a 5)))
  (fork-thread (lambda () (repeat 'b 3)))

  (terminate-thread)

  ; Output: abababaa


  ; To get preemptive multitasking with Gambit-C, remove the call to
  ; (switch-thread) inside repeat and just before the final call to
  ; (terminate-thread) add:
  ; (##add-timer-interrupt-job switch-thread)

HTH
Roland