Re: `append' vs. `nconc'

[Pascal J. Bourguignon]

Emanuel Berg <embe8573@student.uu.se> writes:

> "Pascal J. Bourguignon" <pjb@informatimago.com>
> writes:
>
>>> OK, so for example this: (setq
>>> completion-ignored-extensions (append
>>> completion-ignored-extensions '(".bcf" ".run.xml"))
>>> ) Is better as: (nconc completion-ignored-extensions
>>> (list ".bcf" ".run.xml")) ?
>>
>> No. You should never use nconc only for effect (just
>> like delete, etc).
>
> What do you mean by "effect"?

Side effect, same thing.

(nconc nil '(1 2 3)) has no side effect!

> The reason I thought it fitting was the definitions of
> `append' and `ncons'. Here I want to change
> `completion-ignored-extensions'. `append' doesn't
> change it, which is why the assignment with `setq' was
> there. If it can't be removed (well, in the form of
> `setf'), there is still the gain of not creating the
> list unnecessarily, which in this case is a matter of
> principle but in other cases (with long lists) is
> a performance bottleneck?
>
>> Always re-assign the modified list:
>>
>>     (setf completion-ignored-extensions (nconc
>> completion-ignored-extensions (list ".bcf"
>> ".run.xml")))
>>
>> otherwise it will break when the place is bound to
>> nil. (nil is a symbol and cannot be mutated into
>> a cons cell!).
>
> ?
>
> What place is bound to nil, when?

Any place you want, when it is.

    (let ((mylist '()))
       (nconc mylist (list 1 2 3))
       mylist)
    --> nil

    (setf completion-ignored-extensions '())
    (nconc completion-ignored-extensions  (list ".o"))
    completion-ignored-extensions
    --> nil


-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk