Re: plist-put: destructive?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Drew Adams <drew.adams@oracle.com> writes:

>> Thanks to you both. I'm working with existing plists, so I'll drop
>> the setqs, that's a relief.
>
> Why is it a relief - a relief from what?  What if something reset your "existing plist" to nil behind your back?

The code I'm working has many repeated clauses looking like:

(when thing1
  (setq the-plist (plist-put the-plist :thing thing1)))

A little more complicated than that, but there will be like seven of
those in a row. Probably I should just write myself a custom
bulk-conditional-plist-setter macro, probably using the gv setter, but
for the time being just being able to remove all the "(setq the-plist"
makes everything easier to read.

> Setting (again) a variable whose value is an object to which you've
> applied a destructive operation is the standard idiom. And it is good
> programming practice. It ensures that your code does what you expect,
> regardless of the object value, or rather, regardless of the effect of
> the operation.
>
> You naturally think of your variable as always pointing to the object that you expect.  To keep this relationship, you must set the variable to the return value of any destructive operation.
>
> This is true for a possibly "destructive" operation on any kind of object.  The poster-child case is a list.  See, for example, (elisp) `Rearrangement' (http://www.gnu.org/software/emacs/manual/html_node/elisp/Rearrangement.html).
>
> The point is that your variable is otherwise not guaranteed to
> continue to point to the object that you think it does. You need to
> set it to the _result_ of the possibly destructive operation (the
> return value), if you really want the variable to take the effect of
> that operation into account.
>
> This is all about the difference between a variable and its object value.

I understand the difference, but in this particular case I'm willing to
drop the setqs.

Eric