Re: [External] : Re: Appending lists

[Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

> Actually `append' can also be trouble because the last list
> isn't copied, the new list just has a cdr somewhere to it -
> yes, to the actual list [...]
>
> I don't know why append does this, either to speed things up
> or it has something to do with the car/cdr dynamic, maybe it
> is considered safe enough to move the car out of action with
> an actual copy ...

(setq list-head '(1)) ; (1)

(setq list-tail '(2 3 4)) ; (2 3 4)

(setq whole-list (append list-head list-tail)) ; (1 2 3 4)

(setcdr list-tail '(3.5 4)) ; (3.5 4)

whole-list ; (1 2 3.5 4) <-- the tail has changed

(setcar list-head 1.5)

whole-list ; <-- but here, the head is still unaffected, (1 2 3.5 4)

(setcar whole-list 1.33) ; well, of course that will work...

whole-list ; (1.33 2 3.5 4)

list-head ; still (1.5)

-- 
underground experts united
https://dataswamp.org/~incal