RE: Adding Lists/Sequences

[Drew Adams]

> >>> explain when it is good (if it is) to use these 
> >>> destructive functions.
> >>
> >> Generally, speed.  Non-destructive operations often create 
> >> or copy a lot of new cons cells.
> >
> > Or when you want to share Lisp structure. A special case is 
> > a circular structure, which shares part(s) of itself with itself.
>
> Can you provide a small example ? 

(I meant "list structure", not "Lisp structure" BTW.)

Real examples are best for this kind of thing. Grep for any of these destructive
functions in the Emacs installation `lisp' directory and its subdirectories:
`nconc', `setcdr', `setcar', `delq', `(delete' (you'll get too many hits for
just `delete'), `nreverse', or `sort' . 

You will see in the code that such functions are used _judiciously_, never
wantonly, and you will likely be able to see why, in context. Sometimes comments
explain the need for structure modification or sharing in a particular context.
If you don't understand the existing code, however, then I'd say stay away from
using list modification in your own code. 

Also, see node `Modifying Lists' and its subnodes in the Elisp manual. They give
how-to examples and explain the use. Likewise, see node `Circular Objects'.

If you really need to modify or share list structure for some reason, you will
probably know about it. ;-) IOW, a real need will lead you to finding out more
about it. If not, it's a good idea to stay away from using it. 

And, other things being equal, it's generally a bad idea to try using
modification just to speed up your code. IOW, don't try substituting `nconc' for
`append' liberally, thinking that you are optimizing things. It is more likely
that you are breaking things. (Of course, breaking things can sometimes be a
good way to learn, but beware that debugging code that is broken because of list
modification can be confusing.)