On Thu, 24 Mar 2005 18:27:28 -0600, Joe Corneli <[EMAIL PROTECTED]> wrote: > The `append' doesn't alter the structure of the list A. > > (defvar *foo* (list 1 2 3)) > (append *foo* (list 4 5 6)) > *foo* => (1 2 3) > > Hence, the result of append doesn't alter A's structure. > > Note the `setq' above, which make it look an awful lot like the > structure of A *is* being modified. I mean, it comes out as a > different list --
The setq modifies what the value of the symbol A is - it's destructive in that sense. After the setq, instead of being (1 2 3), A's value to the list (1 2 3 4). But it's a different list (in memory) from its former value. > So, just restrict yourself to destructive operations on A - like > setcdr, setcar, etc. - and you'll be set. Just note that A will always > have to be the "tail" part of B. > > OK, I think I've got the idea now. But still, I'm surprised that `setq' > is not among the list of "destructive functions". What's that about? Like I said, the difference is in just _what_ it's destructively modifying. In the case above, it destructively modifies the value of the symbol A (see `symbol-value'), not the structure of the list whose head cons cell is the value of the symbol A (which is just a more precise way of saying "the list A"). -- Denis Bueno PGP: http://pgp.mit.edu:11371/pks/lookup?search=0xA1B51B4B&op=index _______________________________________________ Help-gnu-emacs mailing list [email protected] http://lists.gnu.org/mailman/listinfo/help-gnu-emacs
