Re: scheme-question about accumulating lists of lists

[David Pirotte]

Hi again,

        Replying twice to myself in a row, how is that :)
        A little tired I guess ...

> > Note that the above will only work if the last 'blue item' has 3 elements, 
> > you'd
> > need to adapt for other use case (which also 'speak' in favor of the cleaner
> > approach.  

> Actually, I didn't like what I wrote, here is a slightly better code:

And here is a corrected version: the code in the mail I'm answering now would 
not
return proper (expected) results for any other operator then car ... this one 
will
let you really walk :)

(use-modules (ice-9 match))

(define blue
  (cons '(a b c) (cons '(1 2 3) '(x y z))))

(define fox
  (cons '(a b c) (cons '(1 2 3) (cons '(x y z) '()))))

(define (blue-walk blue proc)
  (let loop ((blue blue)
             (result '()))
    (match blue
      ((a . rest)
       (if (pair? a)
           (loop rest
                 (cons (proc a) result))
           (reverse! (cons (proc (cons a rest))
                           result))))
      (()
       (reverse! result)))))

scheme@(guile-user)> (load "blue.scm")
;;; note: source file /usr/alto/projects/guile/blue.scm
;;; ...
scheme@(guile-user)> (blue-walk blue car)
$2 = (a 1 x)
scheme@(guile-user)> (blue-walk fox car)
$3 = (a 1 x)
scheme@(guile-user)> (blue-walk blue cdr)
$4 = ((b c) (2 3) (y z))
scheme@(guile-user)> (blue-walk fox cdr)
$5 = ((b c) (2 3) (y z))

Cheers,
David

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