bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

[Ulrich Müller]

Section 13.9 "Accessing Function Cell Contents" of the GNU Emacs Lisp
Reference Manual emphasizes the distinction between void and nil
in function cells:

|    Note that void is not the same as ‘nil’ or the symbol ‘void’.
| The symbols ‘nil’ and ‘void’ are Lisp objects, and can be stored into
| a function cell just as any other object can be (and ‘void’ can be a
| valid function if you define it with ‘defun’).  A void function cell
| contains no object whatsoever.

|    You can test the voidness of a symbol's function definition with
| ‘fboundp’.  After you have given a symbol a function definition, you
| can make it void once more using ‘fmakunbound’.

Also, for "fboundp":

|      This function returns ‘t’ if the symbol has an object in its
|      function cell, ‘nil’ otherwise.  It does not check that the
|      object is a legitimate function.

It seems that the actual behavior does not reflect this, i.e. there
is no distinction between nil and void:

   (fmakunbound 'foo)
   (fboundp 'foo) ⇒ nil

   (fset 'foo nil)
   ;; according to the manual, the following should return t
   ;; because nil is a Lisp object:
   (fboundp 'foo) ⇒ nil

Is the manual wrong, or am I missing something?

bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

[Eli Zaretskii]

> From: Ulrich Müller <ulm@gentoo.org>
> Date: Sat, 19 Oct 2024 16:37:33 +0200
> 
> Section 13.9 "Accessing Function Cell Contents" of the GNU Emacs Lisp
> Reference Manual emphasizes the distinction between void and nil
> in function cells:
> 
> |    Note that void is not the same as ‘nil’ or the symbol ‘void’.
> | The symbols ‘nil’ and ‘void’ are Lisp objects, and can be stored into
> | a function cell just as any other object can be (and ‘void’ can be a
> | valid function if you define it with ‘defun’).  A void function cell
> | contains no object whatsoever.
> 
> |    You can test the voidness of a symbol's function definition with
> | ‘fboundp’.  After you have given a symbol a function definition, you
> | can make it void once more using ‘fmakunbound’.
> 
> Also, for "fboundp":
> 
> |      This function returns ‘t’ if the symbol has an object in its
> |      function cell, ‘nil’ otherwise.  It does not check that the
> |      object is a legitimate function.
> 
> It seems that the actual behavior does not reflect this, i.e. there
> is no distinction between nil and void:
> 
>    (fmakunbound 'foo)
>    (fboundp 'foo) ⇒ nil
> 
>    (fset 'foo nil)
>    ;; according to the manual, the following should return t
>    ;; because nil is a Lisp object:
>    (fboundp 'foo) ⇒ nil
> 
> Is the manual wrong, or am I missing something?

I think the manual is wrong.  I think it tries to explain wrt void
functions the same it says about void variables, but we handle void
variables differently from void functions: void variables have a
distinct value in their value cell, whereas void functions have nil in
their function cell.

Stefan, am I right?

bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

[Andreas Schwab]

On Okt 19 2024, Ulrich Müller wrote:

> Is the manual wrong, or am I missing something?

That changed in commit eadf1faa3cb, but the manual wasn't updated.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

[Ulrich Müller]

>>>>> On Sat, 19 Oct 2024, Andreas Schwab wrote:

> On Okt 19 2024, Ulrich Müller wrote:
>> Is the manual wrong, or am I missing something?

> That changed in commit eadf1faa3cb, but the manual wasn't updated.

Then the respective docstrings are also outdated, i.e. the term "void"
shouldn't occur in any of these:

   (fboundp SYMBOL)
   Return t if SYMBOL’s function definition is not void.

   (fmakunbound SYMBOL)
   Make SYMBOL’s function definition be void.
   Return SYMBOL.

   (symbol-function SYMBOL)
   Return SYMBOL’s function definition, or nil if that is void.