Re: Is it safe to modify a property list directly with PLIST-PUT?

[Pascal J. Bourguignon]

Teemu Likonen <tlikonen@iki.fi> writes:

> I use a list of property lists to store data. It's like this:
>
>     (setq my-data '((:foo "one" :bar "two")
>                     (:foo "three" :bar "four")))

It is never safe to modify literal data!

     (setq my-data (list (list :foo "one" :bar "two")
                         (list :foo "three" :bar "four")))
or:
     (setq my-data (copy-tree '((:foo "one" :bar "two")
                                (:foo "three" :bar "four"))))


> Sometimes I need to modify the data and a command like this seems to
> work:
>
>     (plist-put (nth 1 my-data) :bar "New value")
>
> That is, PLIST-PUT modifies the property list and variable MY-DATA
> contains now the modified list:
>
>     ((:foo "one" :bar "two")
>      (:foo "three" :bar "New value"))
>
> The question: Is this reliable? Is it guaranteed that it will always
> modify the list correctly? If not, how would you suggest doing it
> instead?

It is safe, as long as the property list is not literal data (that
must be considered immutable).

Notice also that like delete, plist-put returns the result, it cannot
always modify the property list in place.  So you have to restore the
result:

   (setf (nth 1 my-data) (plist-put (nth 1 my-data) :bar "New value"))


> Common Lisp has so nice SETF macro...

Of course, emacs has it too:

   (require 'cl) ; put that in your ~/.emacs


-- 
__Pascal Bourguignon__