Re: "The starting list count" ?????

[Alan Mackenzie]

Hello, Robin.

On Mon, Jan 03, 2022 at 12:00:29 -0600, Robin Tarsiger wrote:
> Alan Mackenzie wrote:
> >     `#N'
> >         When printing circular structures, this construct is used to
> >         represent where the structure loops back onto itself, and `N' is
> >         the starting list count:

> >              (let ((a (list 1)))
> >                (setcdr a a))
> >              => (1 . #0)

> > ..  What does this mean, please?  What does "is the starting list count"
> > mean?  There is only one "list", so what is the "list count"?

> I agree that this is a bit too abbreviated, but basically it's the number
> you'd pass to nthcdr along with the immediately enclosing list to get what's
> spliced in as that last cdr.

Thanks, that's helpful.

>     (a b c . #0) ~ (a b c a b c a b c ...)
>     (a b c . #1) ~ (a b c b c b c ...)
>     (a b c . #2) ~ (a b c c c ...)
>     (a b c . #3) ~ <can't happen>

But what about the (1 . #1) I'm seeing?  Isn't that the <can't happen>
case?  The #1 seems to be "pointing to itself".

I see this in the backtrace after C-g'ing out of the loop.

I get an infinite loop when (1 . #1) gets passed to copy-tree.

> -RTT

-- 
Alan Mackenzie (Nuremberg, Germany).