HELP PROCESS                                 John Williams, January 1988


         CONTENTS - (Use <ENTER> g to access required sections)

  1   Introduction
  2   Process Handling Functions
  3   An Example


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1  Introduction
-----------------------------------------------------------------------

The PROCESS  module provides  a  Lisp interface  to the  Poplog  process
mechanism. A process  in Poplog  is a  data structure  that records  the
state  of  execution  of  a  piece  of  Poplog  program.  A  process  is
constructed  from  a  function  and  some  arguments;  it  may  be  run,
suspended,  resumed  or  killed.  The  Poplog  process  facilities   are
described fully in the Pop-11 documentation file REF * PROCESS.

To load the PROCESS module, type

    (require :process)

Process objects have the type  pop11::process. All processes created  by
Lisp are labelled with a unique integer, which is displayed on  printing
a process  object,  and also  is  returned  as the  result  of  applying
process-p to a process.


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2  Process Handling Functions
-----------------------------------------------------------------------

 (Note: In the descriptions below, any argument form termed process
  must either be a process, or the value t, in which case the value
  of *process* will be used)


*process*                                                     [variable]
        This variable holds the currently  active process; or nil if  no
        processes are active. Users may not assign to it.


(make-process function &rest args)                            [function]
        Constructs a process that when run will apply function to  args.
        args need  not  be  all  the arguments  to  function  -  further
        arguments can be passed to function by run-process (see below).


(make-process-to function)                                    [function]
        Constructs a  process  that records  the  state of  the  current
        procedure calling sequence, from the call of make-process-to  up
        to and  including the  most recent  call of  function. When  the
        process  is   run,   it  first   returns   from  the   call   of
        make-process-to, with nil as result.


(run-process process &rest values)                            [function]
        Runs process. If run-process is called while another process  is
        running, then  process  is run  as  a sub-process:  the  calling
        process is not swapped out. If process has not yet been run, the
        values are passed as arguments to the function that process will
        invoke. Otherwise  values  become the  results  of the  call  of
        suspend-process by which process was last suspended.

        Note: If a return-from or throw is executed that crosses  an
        invocation of run-process, then the process being run is killed.
        Also, the wrong  number of  results may  be passed  back to  the
        enclosing block or catch.


(suspend-process process &rest values)                        [function]
        Suspends process, returning values from the call of  run-process
        or (k)resume-process which activated process.


(ksuspend-process process &rest values)                       [function]
        Like suspend-process; in addition, process is killed.


(resume-process process new-process &rest values)             [function]
        Suspends process,  and  activates new-process.  The  values  are
        interpreted in the same manner as for run-process.


(kresume-process process new-process &rest values)            [function]
        Like resume-process; in addition, process is killed.


(copy-process process)                                        [function]
        Returns a copy of process.


(process-p object)                                            [function]
        If object  is  a process,  returns  an integer  identifying  the
        process  (or  t  if  the   process  wasn't  created  by   either
        make-process or make-process-to). Returns nil if object is not a
        process.


(live-process-p process)                                      [function]
        Returns process if process is currently active, t if process  is
        runnable but currently inactive, and nil if process is dead.


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3  An Example
-----------------------------------------------------------------------

;;; Define a function fringe-equal, to determine whether the fringes of
;;; two arbitrarily nested list structures are equal. To solve this
;;; problem without processes, both trees must be "flattened" in
;;; advance, which is expensive if they differ in some of the first
;;; accessible leaves. The method used here involves creating a process
;;; for each tree that recursively descends through the tree structure,
;;; suspending itself and returning each leaf as it is found.

(require :process)

(defun nextleaf (tree)
    (if (listp tree)
        (mapc #'nextleaf tree)
        (suspend-process t tree)))

(defun leaves (tree)
    (nextleaf tree)
    (ksuspend-process t nil))

(defun fringe-equal (t1 t2)
    (do* ((p1 (make-process #'leaves t1))
          (p2 (make-process #'leaves t2))
          (l1 nil (run-process p1))
          (l2 nil (run-process p2)))
         ((or (not (eq l1 l2))
              (not (and (live-process-p p1) (live-process-p p2))))
          (eq l1 l2))))


;;; Now try

(fringe-equal '(a (b (c (d (e)))))
              '(((((a) b) c) d) e))


;;; Also, try tracing nextleaf.


--- C.all/lisp/help/process
--- Copyright University of Sussex 1987. All rights reserved.