Parallel amb (Chicken Scheme)

parallel-amb.scm

(define-syntax amb-parallel
  (syntax-rules ()
    ((_ arg1 arg2)
     (receive (pipefd0 pipefd1) (create-pipe)
              (let ((pid (process-fork)))
                   (if (eq? pid 0)
                       (let ((pipefd1_port (open-output-file* pipefd1)))
                            (write (amb-collect arg2) pipefd1_port) ;we can't backtrack into parent so we collect all of this level's results
                            (close-output-port pipefd1_port)
                            (exit 0)) ;child is done
                       (let* ((res1 (amb-collect arg1)) ;we cannot backtrack here either, the child counts on the parent, so we collect all of this level's results
                              (res2 (read (open-input-file* pipefd0)))) ;after we collected our results we can collect the result of the child
                             (if (and (null? res1) (null? res2)) ;but from here we can backtrack safely yet again
                                 ((amb-failure-continuation))
                                 (if (null? res1)
                                     (amb1 res2)
                                     (if (null? res2)
                                         (amb1 res1)
                                         (amb1 (list (amb1 res1) (amb1 res2)))))))))))
    ((_ arg1 arg2 argi ...)
     (amb1 (amb-parallel (amb-collect arg1) (amb-collect (amb-parallel arg2 argi ...)))))))