Re: Weird Guile Scheme Behaviour

[Neil Jerram <neiljerram@gmail.com>]

Correct usage of append! usually requires (set! x (append! x ....)),
despite what you might think from the !.  I haven't read the rest of your
email carefully, but I wonder if that will make a difference?

Best wishes,
    Neil


On Fri, 13 Sep 2019 at 13:39, Philip K. <philip@warpmail.net> wrote:

>
> Hi,
>
> I was reading a thread on an imageboard[0] the other day, then I came
> across a most peculiar "bug", if it even is one. Since the original
> example was a bit dense (it tried to solve a problem someone else had
> posted, that's not relevant here), I tried to construct a minimal
> working example to discuss here.
>
> Compare
>
>     (define (reverse-iota-1 max)
>       (let ((numbers '(start)))
>         (let loop ((val 0))
>           (append! numbers
>                    (if (< max val)
>                        '()
>                        (begin
>                          (loop (1+ val))
>                          (list val)))))
>         numbers))
>
> and
>
>     (define (reverse-iota-2 max)
>       (let ((numbers '(start)))
>         (let loop ((val 0))
>           (append! numbers
>                    (if (< max val)
>                        '()
>                        (begin
>                          (loop (1+ val))
>                          (list val)))))
>         (cdr numbers)))
>
> (I know, the style is horrible, but that's not the point. Also, both
> have an internal state, so you have to re-eval the function every time
> before starting the function itself.)
>
> The only difference is in the last line. The first function returns the
> entire list (with the start symbol), and the second tries to chop it
> off.
>
> But what happens is that (reverse-iota-1 4) evals to '(start 3 2 1 0)
> while (reverse-iota-2 4) just returns '()!
>
> This seems weird, since my intuition, and that of the poster above, was
> that all that should change in reverse-iota-2 is that the "start" symbol
> should fall away.
>
> It's obvious that this has something to do with the destructive
> "append!", but I'm not quite sure what leads to this unexpected result.
> Is it maybe a optimisation error? Any opinions?
>
> [0]: https://lainchan.org/%CE%BB/res/12185.html#15066 (SFW)
>
> --
>         With kind regards,
>         Philip K.
>