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

[B. Pym]

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.
* (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)

[B. Pym]

Post by B. Pym

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.
* (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)