RE: plist-put: destructive?

[Drew Adams <drew.adams@oracle.com>]

> 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?

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.