Re: Adding Lists/Sequences

[Thierry Volpiatto]

David Kastrup <dak@gnu.org> writes:

> Thierry Volpiatto <thierry.volpiatto@gmail.com> writes:
>
>> Nordlöw <per.nordlow@gmail.com> writes:
>>
>>> Is there a general function, say foo, that adds lists or, even better,
>>> sequences together?
>>>
>>> I want this
>>>   (foo '("a" "b") '("c" "d"))
>>> to evaluate to
>>>   '("a" "b" "c" "d")
>>
>> ,----
>> | ELISP> (nconc '("a" "b" "c") '("d" "e" "f")) 
>> | ("a" "b" "c" "d" "e" "f")
>> `----
>>
>> Note: `append' is not destructive
>
> This is so bad that I can't believe it.
>
> (defun ugh () (nconc '("a" "b" "c") '("d" "e" "f")))
> (ugh) -> ("a" "b" "c" "d" "e" "f")
> (ugh) -> ("a" "b" "c" "d" "e" "f" . #3)
>
> The latter is a tail-cyclic list.
>
> (ugh) -> hangs
We speak of what is bad and very bad but didn't answer to what is good
or not, i send here a bad exemple as i constat i never used that in my
programs (nconc), idem for `nreverse'.And when i read again my commonlisp
book it say that is very delicate to use destructives functions.
So can you explain when it is good (if it is) to use these destructive
functions.

Here an example of the use of append that is safe.


,----
| ELISP> (setq A '(a b c))
| (a b c)
| 
| ELISP> (setq B '(1 2 3))
| (1 2 3)
| 
| ELISP> (defun quite-good (beg tail)
|          (append beg tail))
| quite-good
| ELISP> (quite-good A B)
| (a b c 1 2 3)
| 
| ELISP> (setq C (quite-good A B))
| (a b c 1 2 3)
| 
| ELISP> A
| (a b c)
| 
| ELISP> B
| (1 2 3)
| 
| ELISP> C
| (a b c 1 2 3)
`----

-- 
A + Thierry Volpiatto
Location: Saint-Cyr-Sur-Mer - France