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Re: Cycle Org Shift Select
From: |
Christopher Dimech |
Subject: |
Re: Cycle Org Shift Select |
Date: |
Tue, 10 Nov 2020 11:17:22 +0100 |
Many thanks Tomas, have gone though the Elisp Manual yesterday
and I am getting to understand this list Ouroboros thing. :)
> Sent: Tuesday, November 10, 2020 at 9:08 AM
> From: tomas@tuxteam.de
> To: help-gnu-emacs@gnu.org
> Subject: Re: Cycle Org Shift Select
>
> On Mon, Nov 09, 2020 at 11:49:33PM +0100, Christopher Dimech wrote:
> > What is the meaning of the '.' in '#1=(nil t always . #1#)?
>
> This means that the last cdr in that list isn't nil, as "decent,
> normal lists" (aka "proper lists") have, but a reference to something,
> in this case to the list's head (#1#), which you have taken previously,
> in #1=. Making the whole thing an Ouroboros [1].
>
> In box-and-pointer-ish, the list '(nil t always) looks like so
> (WARNING: psychedelic effects guaranteed when using variable-pitch
> fonts. But who does that, anyway?):
>
> +-----+--+ +--+--+ +--+-----+
> | nil | | ---> | | | ---> | | nil |
> +-----+--+ +--+--+ +--+-----+
> | |
> v v
> 't 'always
>
> Note how the list ends with a pair whose cdr is nil.
>
> The dot notation allows you to fill both slots of a pair (car
> and cdr), thus putting something other than the "link to next"
> in a cdr, creating an improper list; the reference notation
> (that #...= and #...#) allows you to take a ref at some place
> and use it later. That's what your above monster looks like
> in box-and-pointer:
>
> +---------------------------------+
> | |
> v |
> +-----+--+ +--+--+ +--+--+ |
> | nil | | ---> | | | ---> | | | ---+
> +-----+--+ +--+--+ +--+--+
> | |
> v v
> 't 'always
>
>
> So if you run *that* list along (by doing cdr), you'll run
> in circles, over and over again. That was this device's
> intention, anyway, if I followed along correctly.
>
> Cheers
>
> [1] https://en.wikipedia.org/wiki/Ouroboros
> - t
>
- Re: Cycle Org Shift Select, (continued)
- Re: Cycle Org Shift Select, Christopher Dimech, 2020/11/09
- Re: Cycle Org Shift Select, Noam Postavsky, 2020/11/09
- Re: Cycle Org Shift Select, Michael Heerdegen, 2020/11/09
- Re: Cycle Org Shift Select, Christopher Dimech, 2020/11/09
- Re: Cycle Org Shift Select, Christopher Dimech, 2020/11/09
- Re: Cycle Org Shift Select, tomas, 2020/11/10
- Re: Cycle Org Shift Select,
Christopher Dimech <=
- Re: Cycle Org Shift Select, Michael Heerdegen, 2020/11/10
RE: Cycle Org Shift Select, Drew Adams, 2020/11/10
Re: Cycle Org Shift Select, Michael Heerdegen, 2020/11/09