Re: The purpose of makunbound

[Kelly Dean <kelly@prtime.org>]

Stefan Monnier wrote:
> Why would that be an error?

Because of a classic mistake with dynamic binding:
(defvar bar nil)
(defun foo ()
  (if something-rare...
      (makunbound 'bar)) ; Intent is global
  (do-something))

(let ((bar (something))) ; Intent is lexical
  (do-some-stuff)
  (foo)
  (if bar ; Oops
      (some-other-stuff)))

Well the classic is with «set», not makunbound, but the problem is the same.

There are legitimate cases for using «set» in dynamic «let», so the interpreter must allow it, and you just have to deal with the possibility of that kind of mistake. But for makunbound, if there are no legitimate use cases, then the interpreter could catch it.

> How often have you bumped into that error?

Never.

> And remember: a (dynamically scoped) `let' does not necessarily mean to
> make a local binding, it can also be understood to mean to temporarily
> change the global variable
[snip]
> You talk about "global variable" vs "dynamic variable", but that
> difference is only in your head because of the interpretation you chose.

For single-threaded code, there's no difference, but for multi-threaded, there is, which you already mentioned. Why have inconsistent interpretations for single vs. multi, when you can have a consistent one for both?

And aren't you planning to add multi-threading to Emacs?