In article <mailman.13637.1526042360.27995.help-gnu-emacs@gnu.org>,
 Noam Postavsky <npostavs@gmail.com> wrote:

> On 10 May 2018 at 22:30,  <rslovers@yandex.com> wrote:
> 
> > Today I accidentally ran (fset 'nil 'message), after that multiple
> > functions like forward-word, kill-word stopped working, and complained
> > that `Symbol’s function definition is void: nil'.
> >
> > After (fset 'nil 'nil) they started working again, does anybody undertand
> > what is going on here?
> 
> Yes, after you do (fset 'nil 'message), (functionp nil) returns t, but
> (funcall nil) still fails. Many places do things like
> 
> (when (functionp some-var)
>   (funcall some-var))
> 
> which will cause the errors you see if `some-var' is nil.
> 
> In Emacs 26, doing (fset 'nil 'message) will fail (see Bug#25110).
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25110

This brings back memories. Several decades ago I worked for a company 
that used Symbolics Lisp Machines, and one of our applications had a bug 
that did something like (set 'nil <something>). This crashed the 
operating system. Turns out that the software checked for this in SETQ, 
but not SET.

Symbolics put the value cells of NIL and T into non-writable memory in 
the next release.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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