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.

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

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> 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.

Oops.  Looks like I missed this part when I changed it back around
Emacs-24.4:

    ** In 'symbol-function', nil and "unbound" are indistinguishable.
    'symbol-function' does not signal a 'void-function' error any more.
    To determine if a symbol's function definition is void, use 'fboundp'.

> I think the manual is wrong.

Indeed.  It was right for Emacs<24.4, tho.


        Stefan

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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Ulrich Müller <ulm@gentoo.org>,
>   73886@debbugs.gnu.org
> Date: Sat, 19 Oct 2024 22:12:48 -0400
> 
> >> 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.
> 
> Oops.  Looks like I missed this part when I changed it back around
> Emacs-24.4:
> 
>     ** In 'symbol-function', nil and "unbound" are indistinguishable.
>     'symbol-function' does not signal a 'void-function' error any more.
>     To determine if a symbol's function definition is void, use 'fboundp'.

Could you explain the rationale for that change?  I tried to look for
relevant discussions around that date, but came up empty-handed.

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

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> Oops.  Looks like I missed this part when I changed it back around
>> Emacs-24.4:
>> 
>>     ** In 'symbol-function', nil and "unbound" are indistinguishable.
>>     'symbol-function' does not signal a 'void-function' error any more.
>>     To determine if a symbol's function definition is void, use 'fboundp'.
>
> Could you explain the rationale for that change?  I tried to look for
> relevant discussions around that date, but came up empty-handed.

I can't remember discussing it, no.  It was a kind of "executive
decision".

Having a special "void" (`Qundefined`) non-value for the `symbol-value`
is needed for `boundp` since variables can contain *any* value, but not
for the `symbol-function` part where we can use any normal value (I
chose `nil`) to play this role as long as it doesn't collide with values
normally held in the `symbol-function` slot, like function names,
function values, cons cells, vectors, ...

The upside was a simplification in various chunks of code which used to
do things like `(and (fboundp SYM) (symbol-function SYM))` which can now
be simplified to `(symbol-function SYM)`.

I remember two "motivators", i.e. places where the need to pay attention
to the special void case annoyed me enough to look into this and make
the change, one was `nadvice.el` and the other was `cl-letf`.
[ So, it was no accident that the change happened in the same release as
  the addition of `nadvice.el`.  ]

In both cases the issue is that we want to deal with "places"
(generalized variables) and that abstraction works well for those places
which *always* contain a value, but not as well for those special places
that can be "unbound", so removing the "unbound" case from
`symbol-function` resulted in a welcome simplification.
For the same reason I dislike EIEIO's notion of `slot-boundp` and have
already considered marking it obsolete.



        Stefan