Re: Bug & co

[Lauri Alanko <la@iki.fi>]

On Wed, Dec 12, 2001 at 03:12:01PM +0100, Jerzy Karczmarczuk wrote:
> How, for goodness' sake, l can guess what happened to the data it points to?
> It continues to point to the *same cell*.
> 
> This is - sorry to appear harsh - an *elementary* fact belonging to what we
> call "call by value" (here: dereferenced argument).

The semantics of reverse! are not _completely_ obvious, especially not
to someone who's used to, say, arrays. For example, the original poster
_might_ have expected reverse! to simple rearrange the _elements_ in the
list without modifying the structure, like this:

(define (reverse! l)
  (define (aux curr)
    (if (null? curr)
        l
        (let ((my (aux (cdr curr))))
          (and my
               (not (eq? my curr))
               (not (eq? my (cdr curr)))
               (let ((tmp (car my)))
                 (set-car! my (car curr))
                 (set-car! curr tmp)
                 (cdr my))))))
  (aux l)
  l)

Of course this is inefficient and generally a bad idea, but still, it's
not "elementary", and certainly not obvious from the description "list
is destructively reversed".


Lauri Alanko
la@iki.fi