18

I've accidentally run the following:

(unintern variable)

where variable's value was nil.

How do I get nil back without restarting Emacs?

  • 3
    +1 for making me chuckle. :-) Not in a condescending way, but in a "I would have totally done that as well" way. – Malabarba Oct 31 '14 at 23:44
  • @Malabarba :) It was truly a humbling realization when I figured out what I had done – Sean Allred Nov 1 '14 at 1:23
  • 1
    You can delete nil! Did not see that coming. – Gambo Nov 1 '14 at 6:58
  • @Gambo neither did I :) – Sean Allred Nov 1 '14 at 13:38
19

(defconst nil ())

seems to have the right effect; note that nil and an empty list are indistinguishable in Emacs Lisp.

I looked at lread.c:4034 to see how nil is created in an obarray.

Note the comment at line 3896 in lread.c:

/* There are plenty of other symbols which will screw up the Emacs
     session if we unintern them, as well as even more ways to use
     `setq' or `fset' or whatnot to make the Emacs session
     unusable.  Let's not go down this silly road.  --Stef  */
  /* if (EQ (tem, Qnil) || EQ (tem, Qt))
       error ("Attempt to unintern t or nil"); */

This explains why Emacs does not protect against (unintern nil) and (unintern t).

  • Very clever! I agree with Stef on this one, actually :) Down that road lies madness. – Sean Allred Oct 31 '14 at 23:37
  • Bleh. I agree with rms over sm here. Uninterning nil is a common error that's easy to protect against. Why put a handrail on a staircase when people could jump over it? – Gilles Oct 31 '14 at 23:55
  • @Gilles I think about this as I think about C: C maintains the philosophy that the programmer knows what he is doing and merely requires a way to express himself. IMO, we should regard emacs lisp as the assembly language of emacs; higher abstractions (and guards/features) should be added in a language that compiles to elisp. But that is certainly a topic for another medium :) – Sean Allred Nov 1 '14 at 0:18
  • unintern is not used frequently, really. Using it on the main obarray is pretty much always a bad idea (which is why the byte-compiler complains when you fail to pass the second argument, and C-h f unintern doesn't tell you that the second arg is actually still optional). – Stefan Nov 1 '14 at 13:12
  • @Stefan It would seem that's a documentation bug, then :( – Sean Allred Dec 7 '14 at 19:51

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