Re: Appending lists

[Jean Louis <bugs@gnu.support>]

* Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> [2021-06-17 01:36]:
> > 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`".

When there is elephant in front of me, and there was one, then
somebody comes across and says, ah, that is not really an elephant in
front of you, it's really nothing more than a reference to a bunch of
walking meat and bones... then there is nothing useful about it. I
have to run anyway, or pee myself.

Or, maybe it would be good changing the Elisp manuals where you can
place the statements like: "the value held in variable `x' is not a
list of 3 elements where by: (let ((x (list 1 2 3))) x) ⇒ (1 2 3) is
not what you see... (irony)

If you wish to go deep, go even deeper and talk how there is no cdr
and car, no cons, and no Lisp, as all we are talking about is about
turning it on and turning it off, you see? There are actually no lists
at all, there is no Lisp, no C programming, as in the reality there is
something else, it is just our consideration... go even deeper and
demand from Emacs Lisp programmer to understand the quantum physics as
in reality, well, there is no matter at all... as what you see is not
really there... go back to the Big Bang, but that is not Emacs Lisp
context.

Emacs Lisp programmer is high level programmer who enjoys the straight
practical language. That is quite a different context.

The value bound to variable `x' makes the variable have that value. As
simple as that.


-- 
Jean

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