Chapter 32 - Dark Corners and Curiosities

This chapter is almost at the end of our survey of Lisp. Here, we'll examine some Lisp features that are newer, unstandardized, experimental, or controversial.

Extended LOOP: Another little language

Chapter 5 described several iterative control forms: DO, DOTIMES, DOLIST, and a simple LOOP. We also saw that FORMAT (Chapter 24) has its own control constructs for iteration.

Recursion is a useful tool for describing (and implementing) some algorithms. But in many cases it's easier to write efficient iterative code than it is to write efficient recursive code. In chapters 4 and 28 we saw how to write tail-recursive code, and learned that Lisp is not required to optimize tail calls. Ironically, iteration is very important in this implementation of a language originally conceived as a notation for recursive functions.

An extended loop facility was introduced late in the specification of Common Lisp. Extended loop, like FORMAT control strings, breaks away from the Lisp tradition of a simple, consistent syntax. Extended loop uses keywords to specify initialization, actions and termination conditions. Here are a few examples:

;; Sum the integers from 1 to 100 
? (loop for n from 1 to 100
        sum n)

;; Compute factorial 10 iteratively 
? (loop for n from 1 to 10
        with result = 1
        do (setq result (* result n))
        finally return result)

;; Gather the even numbers from a list 
? (loop for item in '(1 5 8 9 7 2 3)
        when (evenp item)
        collect item)
(8 2)

Extended loop inspires heated disagreements among Lisp users. Its detractors point out that the behavior is underspecified for complex combinations of options, while its supporters point out that extended loop forms are easier to read than most DO forms for simple operations. You should heed the advice of both camps: use extended loop to improve readability of simple looping operations.

TAGBODY: GO if you must

Ever since the structured programming revolution of the 1970's, programmers and language designers alike have been apologetic about the GOTO construct. Yet there are rare cases where a well-placed GOTO, used with careful consideration, is the clearest way to structure the control flow of an algorithm.

Lisp retains a GOTO as a GO form, but it must be used within the lexical scope of a TAGBODY form. A TAGBODY may contain Lisp forms and symbols. The forms are evaluated, while the symbols (which in other forms might be evaluated for their lexical binding or SYMBOL-VALUE) are simply used as labels to which a GO form may transfer control.

Processes & Stack Groups: Juggling multiple tasks

Leading-edge Lisp systems on dedicated hardware, and more recently on the Unix platform, have implemented a feature called "lightweight processes." In the C world these are known as "threads."

Lightweight processes allow you to write pieces of code which share the CPU's time along with all of the global variables in your LISP environment. Although this is a limited form of multitasking, lacking protection between processes, it is very useful for handling computations which must run "in the background" or in response to asynchronous events.

In the last few years, low-cost Lisp systems have started to include a process facility. Of all the vendors of low-cost Lisp system, Digitool was the first to include processes in its product. Starting with its 4.0/3.1 release, MCL includes a complete implementation of lightweight processes including a full range of control, synchronization, and communication abstractions. MCL's process API is very close to the API used on the Lisp machines. I'll use MCL's API to illustrate the rest of this section.

The MCL processes are fully preemptive -- you can set both priority and time slice (the "quantum") for each process. Each process can have private variables simply by using local variables in the process run function (i.e., Lisp "closures"). As you'll probably have a need to access shared data as well, the MCL process facility provides locks ("mutexes") to ensure access to critical data by only one process at a time; this is especially useful when multiple fields of a complex structure must be updated in a single operation ("atomically").

The following code implements a solution to Dijkstra's "dining philosophers" problem using MCL processes and locks. In case you're not familiar with this, imagine a group of philosophers seated around a round table. Each philosopher has a plate of food. The food can only be eaten if a philosopher holds a fork in each hand. There is a fork between each pair of philosophers, so there are exactly as many forks as there are philosophers. The objective is to make the philosophers behave so that they all get a fair chance to eat. The classic solution imposes a protocol on how resources (forks) are acquired, in order to prevent deadlock (starvation).

(defstruct philosopher
  (amount-eaten 0)
  (task nil))

(defmacro acquire-lock-or-skip (lock post-acquire pre-release &body body)
     ;; Random sleep makes the output more interesting 
     ;; by introducing variability into the order of 
     ;; execution.  This is a simple way of simulating 
     ;; the nondeterminacy that would result from having 
     ;; additional processes compete for CPU cycles. 
     (sleep (random 5))
     (unless (lock-owner ,lock)
       (process-lock ,lock)
         (progn ,@body)
         (process-unlock ,lock)))))

(let ((philosophers #())
      (philosophers-output t))

  (defun dining-philosophers (number-of-philosophers &optional (stream t))
    (unless (equalp philosophers #())
    (assert (> number-of-philosophers 1) (number-of-philosophers))
    (setq philosophers-output stream)
    (format philosophers-output
            "~2&Seating ~D philosophers for dinner.~%"
    (force-output philosophers-output)
    (flet ((announce-acquire-fork (who fork)
             (format philosophers-output 
                     "~&Philosopher ~A has picked up ~A.~%" 
                     who (lock-name fork)))
           (announce-release-fork (who fork)
             (format philosophers-output 
                     "~&Philosopher ~A is putting down ~A.~%" 
                     who (lock-name fork)))
           (eat (who)
             (format philosophers-output 
                     "~&Philosopher ~A is EATING bite ~D.~%"
                     who (incf (philosopher-amount-eaten (aref philosophers who))))))
      (flet ((philosopher-task (who left-fork right-fork)
                 (acquire-lock-or-skip left-fork
                                       (announce-acquire-fork who left-fork)
                                       (announce-release-fork who left-fork)
                   (acquire-lock-or-skip right-fork 
                                         (announce-acquire-fork who right-fork)
                                         (announce-release-fork who right-fork)
                     (eat who)))
                 (force-output stream)
        (let ((forks (make-sequence 'vector number-of-philosophers)))
          (dotimes (i number-of-philosophers)
            (setf (aref forks i) (make-lock (format nil "fork ~D" i))))
          (flet ((left-fork (who)
                   (aref forks who))
                 (right-fork (who)
                   (aref forks (mod (1+ who) number-of-philosophers))))
            (setq philosophers (make-sequence 'vector number-of-philosophers))
            (dotimes (i number-of-philosophers)
              (setf (aref philosophers i)
                     :task (process-run-function (format nil "Philosopher-~D" i)
                                                 (left-fork i) 
                                                 (right-fork i)))))))))

  (defun stop-philosophers ()
    (dotimes (i (length philosophers))
      (process-kill (philosopher-task (aref philosophers i))))
    (format philosophers-output 
            "~&Dinner is finished. Amounts eaten: {~{~D~^, ~}}~2%" 
            (map 'list #'philosopher-amount-eaten philosophers))
    (force-output philosophers-output)
    (setq philosophers #())

If you evaluate (dining-philosophers 5) and look through the actions of any one philosopher, you'll see her repeatedly do one of two things:

  1. pick up a fork (the left one) and put it down again because the other (right) fork was in use, or
  2. pick up each fork (left, then right), eat, then put down the forks.

When you evaluate (stop-philosophers) you'll see a list of how many times each philosopher has eaten. These numbers will be fairly close to each other, illustrating the fairness of the algorithm.

MCL also exposes a ``stack group'' abstraction, which is useful for implementing coroutines:

;;; Main routine F-FOO 
(defun f-foo ()
  (print 'foo-1)
  (funcall sg-bar nil)        ; call 1 to coroutine 
  (print 'foo-2)
  (funcall sg-bar nil)        ; call 2 to coroutine 
  (print 'foo-3)
  (funcall sg-bar nil)        ; call 3 to coroutine 

;;; Create a stack group for the coroutine. 
(defvar sg-bar (make-stack-group "bar"))

;;; Coroutine F-BAR 
(defun f-bar ()
  (print 'bar-1)              ; do this for call 1 
  (stack-group-return nil)    ; return from call 1 
  (print 'bar-2)              ; do this for call 2 
  (stack-group-return nil)    ; return from call 2 
  (print 'bar-3)              ; do this for call 3 
  (stack-group-return nil)    ; return from call 3 

;;; Initialization and startup 
(defun run-coroutines ()
  ;; Initialize the coroutine
  (stack-group-preset sg-bar #'f-bar)
  ;; Start main coroutine

When you run the main routine, its execution is interleaved with the coroutine:

? (run-coroutines)

You can easily run any function within a separate lightweight process, allowing other computation, compilation, editing, etc. to happen concurrently:

(process-run-function "Annoy me" 
                      #'(lambda (delay)
                            (sleep delay)

Series: Another approach to iteration and filtering

Series were formally introduced with the printing of Common Lisp: The Language (2nd ed) (also known as CLtL2), but were not adopted as part of the ANSI Common Lisp standard. Still, some Lisp vendors include series in their product because customers came to depend upon it during the time between the publication of CLtL2 and the ANSI Specification.

Series combine the behaviors of sequences, streams and loops. Using series, you can write iterative code using a functional notation. Control is achieved by selecting or filtering elements as they pass through a series of filters and operators.

The best place to find information and examples is in Appendix A of CLtL2.

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Copyright © 1995-1999, David B. Lamkins
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