On Sat, May 28, 2022 at 11:45:26PM +0200, Ralf Hemmecke wrote:
> Does anyone know why the original developers did not allow to copy a cyclic
> list?
>
> I guess, one could implement a copy that is again a cyclic list.
>
> And there seems to be a misbehaviour. If I construct l2 in the same way as I
> construct l below, then
>
> (l=l2)@Boolean
>
> runs forever. Since a list is a finite datastructure (i.e. finitely many
> list cells) we should be able to recognize equality. We just need to figure
> out where the cycle begins. Am I wrong?
>
> Ralf
>
> (41) -> l := [1,2,3]
>
> (41) [1, 2, 3]
> Type: List(PositiveInteger)
> (42) -> setrest!(rest(l,2), l)
>
> _______
> (42) [1, 2, 3]
> Type: List(PositiveInteger)
> (43) -> copy l
>
> >> Error detected within library code:
> cyclic list
>
> copy x ==
> y := empty()
> for k in 0.. while not empty? x repeat
> k = cycleMax and cyclic? x => error "cyclic list"
> y := concat(first x, y)
> x := rest x
> reverse! y
>
AFAICS there are pragmatic decisions based on execution time and
expected use cases. Years ago we had similar disscussion about
'length'.
--
Waldek Hebisch
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