Re: Appending lists

[tomas]

On Wed, Jun 16, 2021 at 12:13:21PM +0300, Jean Louis wrote:
> * tomas@tuxteam.de <tomas@tuxteam.de> [2021-06-16 10:29]:
> > On Wed, Jun 16, 2021 at 03:11:43AM +0200, Emanuel Berg via Users list for 
> > the GNU Emacs text editor wrote:
> > > tomas wrote:
> > > 
> > > > (setq thing (copy-sequence '(one two three four five six)))
> > > > (setq thang (cddr thing))
> > > >
> > > > thang => (three four five six)
> > > >
> > > > (nreverse thing) => (six five four three two one)
> > > >
> > > > thing => (one)
> > > >
> > > > thang => (three two one)
> > > >   ; now this is what I was after. Who the heck "changed my
> > > >   ; variable!?" Who is General Failure and why is he reading
> > > >   ; my disk?
> > > 
> > > Well, let's see, `nreverse' has updated the data without
> > > setting the variables to whatever desired values they should
> > > take
> > 
> > It can't. It's a function.
> > 
> > Doing (foo x y) will *never* change "the variable x" -- unless
> > foo is a macro/special form.
> 
> (setq list-1 '(1 2 3))
> (setq list-2 '(A B))
> (nconc list-1 list-2) ⇒ (1 2 3 A B)
> list-1 ⇒ (1 2 3 A B)
> nconc is a built-in function in ‘C source code’.
> 
> So it obviously changes the variable `list-1' from (1 2 3) to (1 2 3 A
> B) -- how do you explain that?

No. A thousand times no. The variable list-1 contained a ref to some
cons cell (behind the scenes it's just a (perhaps tagged) pointer).

**THIS VARIABLE HASN'T CHANGED** after you did that nconc. The variable
list-1 still contains a ref to exactly the same cons cell.

What happens is that you changed the list somewhere downstream.

> So far I know from Common Lisp those functions starting with "n" are
> common with side effects.

Judging by what you write, I have the impression that you haven't
quite understood Common Lisp (this part works exactly the same
there). But I might be wrong and this a huge misunderstanding.

Cheers
 - t

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