Re: P08 (**) Eliminate consecutive duplicates of list elements.

B. Pym wrote:

If a list contains repeated elements they should be replaced
with a single copy of the element. The order of the elements
should not be changed.
 
  Example:
  * (compress '(a a a a b c c a a d e e e e))
  (A B C A D E)

 
In newLisp, "apply" can be used for reduce or fold.
 
(define (compress lst)
  (reverse
    (apply
      (fn (accum x)
        (cond ((empty? accum) (list x))
              ((= x (first accum)) accum)
              (true (cons x accum))))
      (cons '() lst)
      2)  ;; How many things to process at a time.
))
 
(compress '(a a a a b c c a a d e e e e))
  ===>
(a b c a d e)

The list could first be converted to monotonic sublists.

(define (monotonic-slices lst key-func (cmp =))
  (let (result '()  tmp '()  old-key 0  new-key 0)
    (dolist (x lst)
      (set 'new-key (key-func x))
      (cond ((empty? tmp) (push x tmp))
            ((cmp new-key old-key) (push x tmp))
            (true (push (reverse tmp) result) (set 'tmp (list x))))
      (set 'old-key new-key))
    (unless (empty? tmp) (push (reverse tmp) result))
    (reverse result)))

(monotonic-slices '(0 2 3 4 5 7) odd?)
  ===>
((0 2) (3) (4) (5 7))

(monotonic-slices '(0 2 3 3 4 8 7 9) or >)
  ===>
((0 2 3) (3 4 8) (7 9))


So the solution to this problem is:

(map first (monotonic-slices '(a a a a b c c a a d e e e e) or))
  ===>
(a b c a d e)