Re: Why doesn't nconc change my variable?

[Pascal J. Bourguignon]

Marcin Borkowski <mbork@wmi.amu.edu.pl> writes:

> On 2014-10-05, at 03:58, Drew Adams wrote:
>
>>> (setq my-list ())
>>> (nconc my-list '("wtf"))
>>> 
>>> and my-list is still nil.  If, OTOH, I do
>>> (setq my-list ())
>>> (setq my-list (nconc my-list '("wtf")))
>>> 
>>> my-list is ("wtf").
>>> 
>>> Why is that so?  I though nconc is supposed to change all its
>>> arguments but the last one.  Is the latter construct a correct
>>> way of adding an element at the end of the list?
>>
>> No, it's not supposed to do that. You discovered just what it
>> does do.  And you could have discovered it earlier by reading
>> some doc.  See the Elisp manual, node `Rearrangement' (found by
>> doing `i nconc' in the manual).
>
> I did read it.  And a few times, for that matter.
>
> You see, one of the worst intellectual mistakes you can make is to have
> a wrong mental model of something.  This seems to be the case here, and
> that's why I really want to grok this issue.
>
>> See also node `Sets and Lists', in particular this (about `delq',
>> but the same idea applies to other destructive list operations):
>>
>>  Note that `(delq 'c sample-list)' modifies `sample-list' to splice
>>  out the third element, but `(delq 'a sample-list)' does not splice
>>  anything--it just returns a shorter list.  Don't assume that a variable
>>  which formerly held the argument LIST now has fewer elements, or that
>>  it still holds the original list!  Instead, save the result of `delq'
>>  and use that.  Most often we store the result back into the variable
>>  that held the original list:
>>
>>      (setq flowers (delq 'rose flowers))
>
> That I didn't read earlier; I've read it now, and still don't get it.
> Sorry.  (Edit: while I was writing the rest of this post, things
> apparently got more clear, but I'll leave the rest, since if I'm right,
> this might be (hopefully) instructive for others reading it, and I might
> even want to save it to post it on my blog.)
>
> Now I have two choices.  One is to install the C source and try to read
> it.  This I cannot do now (time constraints, and while I did learn some
> C, it was, what, fifteen years ago or so).  That's why I'm asking here.
> Please bear with me.  This should also be good for Emacs, since it may
> be the case that if I cannot understand the manual, someone else might
> have troubles with that, too, so maybe the manual is buggy.
>
> What I thought was essentially this: under the hood, a list is a pointer
> (using C terminology) to a cons cell.  The car of the cons cell is a
> pointer to some data (say, a string), and the cdr - a pointer to the
> next cons cell.
>
> What I thought is that if you say, e.g.,
>
> (setq my-list ("hello" "world"))
>
> and then
>
> (nconc my-list '("wtf"))
>
> then my-list points to the list ("hello" "world" "wtf").  This seems to
> be indeed the case.  (And the wtf's get added if I repeat the nconc step
> - I confirmed that experimentally.)

You did not.  Your experience was wrong or incomplete.

    (setf print-circle t)
    (defun strange (list) (nconc list '("wtf")))
    (strange (strange (list 1 2 3)))

    --> (1 2 3 . #1=("wtf" . #1#))


    (setf print-circle nil
          print-length 15
          eval-expression-print-length 15)
    (defun strange (list) (nconc list '("wtf"))) ; you need to redefine it.
    (strange (strange (list 1 2 3)))
    --> (1 2 3 "wtf" "wtf" "wtf" . #3)

Actually, if you try to call strange^3, it falls into an infinite loop,
because after strange^2, "wtf" is in a circular list.


Again, do not mix immutable data with mutable data, if you want to
mutate the result later!

Use: (nconc list (list "wtf")), not quote.

-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
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