Re: Appending lists

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

> To put a point across it would be better to announce or make a title
> that you don't talk how "variable never changes", but you talk about
> the structures in memory and somewhat interesting though practically
> within the spoken context useless information. What matters for
> programmer is that variable A instead of holding value 1 now has value
> 2, that is a change, and how that change was internally accomplished
> does not change the fact that A is now not 1 but is 2.  (づ｡◕‿‿◕｡)づ

That's the problem with mutation.
In a language like Haskell where mutation is not allowed, when you have

    x = [1, 2, 3]

then the value held in variable `x` is really a list of 3 elements.
But in a language like ELisp, when you have

    (let ((x (list 1 2 3))) ...

the value held in variable `x` is not a list of 3 elements: it's really
nothing more than a reference to a location in memory holding a cons
cell (constraints in ELisp's type system ensure that this location in
memory will always hold a cons cell).  The values held in the `car/cdr`
of that cons cell will depend on the current memory state, they are not
part of "the value held in `x`".

The vast majority of ELisp code never uses `setcar/setcdr`, tho, so we
tend to overlook this inconvenient truth and talk (and think) about
those values as if we were in the presence of immutable data.
That's why `nconc`, `nreverse`, `sort`, and other similar side-effecting
functions need to be used with a lot of care.


        Stefan