bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Jean Louis]

When I evaluate following it is not detecting variable `direction'
under lexical bindings:

(defun verify-direction (timestamp &optional direction)
  (cond ((bound-and-true-p direction) (message "OK"))))

(verify-direction "10:00" t) ⇒ nil

I would like to understand if it is intended to be so? 

Or is this possible bug?



In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, cairo version 1.17.4)
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-- 
Thanks,
Jean Louis

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bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Phil Sainty]

On 2021-06-19 23:39, Jean Louis wrote:
> When I evaluate following it is not detecting variable `direction'
> under lexical bindings:
> 
> (defun verify-direction (timestamp &optional direction)
>   (cond ((bound-and-true-p direction) (message "OK"))))
> 
> (verify-direction "10:00" t) ⇒ nil
> 
> I would like to understand if it is intended to be so?
> 
> Or is this possible bug?

This is expected / not a bug (and so I'm closing it).

(bound-and-true-p VAR) is syntactic sugar for (and (boundp 'VAR) VAR)
which means that it works only for dynamic variables (i.e. variables
stored in the value slot of a symbol).

Under lexical binding, function arguments are lexical variables, which
means they are not stored in the symbol of that name, and hence cannot
be detected with `boundp'.

In your case there's no need to be checking whether `direction' is
"bound" -- whether it's lexical or dynamic, you already *know* that
it exists because it's an argument for the function in which you're
checking it.  All you want is the "true-p" part, which is implicit
in every value (either nil or non-nil).

Instead of "(bound-and-true-p direction)" just use "direction".


-Phil

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Eli Zaretskii]

> Date: Sun, 20 Jun 2021 00:49:14 +1200
> From: Phil Sainty <psainty@orcon.net.nz>
> Cc: 49116-done@debbugs.gnu.org
> 
> (bound-and-true-p VAR) is syntactic sugar for (and (boundp 'VAR) VAR)
> which means that it works only for dynamic variables (i.e. variables
> stored in the value slot of a symbol).
> 
> Under lexical binding, function arguments are lexical variables, which
> means they are not stored in the symbol of that name, and hence cannot
> be detected with `boundp'.

Should this be mentioned in the doc string of bound-and-true-p?

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> Should this be mentioned in the doc string of bound-and-true-p?

I think so.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Phil Sainty]

On 2021-06-20 00:54, Eli Zaretskii wrote:
> Should this be mentioned in the doc string of bound-and-true-p?

It wouldn't hurt, and I see that `boundp' has such a note:

"Note that if `lexical-binding' is in effect, this refers to the
global value outside of any lexical scope."

That same text could be used.

If the confusion is over whether function arguments are dynamic
or lexical then the doc change might not explain things, but it
would probably hint at the source of the confusion.


-Phil

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Lars Ingebrigtsen]

Phil Sainty <psainty@orcon.net.nz> writes:

> It wouldn't hurt, and I see that `boundp' has such a note:
>
> "Note that if `lexical-binding' is in effect, this refers to the
> global value outside of any lexical scope."
>
> That same text could be used.

I've now copied over that text to the doc string of bound-and-true-p.

> If the confusion is over whether function arguments are dynamic
> or lexical then the doc change might not explain things, but it
> would probably hint at the source of the confusion.

Yup.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Michael Heerdegen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> I've now copied over that text to the doc string of bound-and-true-p.

You now changed the text to this:

> Return the value of symbol VAR if it is bound, else nil.
> Note that if `lexical-binding' is in effect, this function isn't
> meaningful if it refers to a lexically bound variable.

If you want feedback: I found this new version confusing, although it's
totally correct.  Because in my mental model, `bound-and-true-p' just
receives a variable (i.e. internally quotes), so how _can_ it even
"refer" to the lexical variable at all?

But it is true:

#+begin_src emacs-lisp
(let* ((a 1)
       (get-a (lambda () (list a (bound-and-true-p a)))))
  (dlet ((a 2))
    (funcall get-a))) ;; -> (1 1), not (1 2) as one might expect
#+end_src

Everything became clear when I looked at the implementation ;-)

Maybe we can further improve the text to something like "not meaningful
in the scope of a lexical binding of the variable" or something like
that?


Thanks,

Michael.

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Lars Ingebrigtsen]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> Maybe we can further improve the text to something like "not meaningful
> in the scope of a lexical binding of the variable" or something like
> that?

But I don't think that's correct, because the function takes a symbol
that refers to a variable -- and not a variable.  :-)

Explaining why something doesn't make sense is often more complicate
than explaining how something works.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#49116: 28.0.50; Why `bound-and-true-p' is not working in lexical binding?

[Michael Heerdegen]

Jean Louis <bugs@gnu.support> writes:

> (defun verify-direction (timestamp &optional direction)
>   (cond ((bound-and-true-p direction) (message "OK"))))
>
> (verify-direction "10:00" t) ⇒ nil

BTW, if your original intention was to check whether a function argument
has been specified in a function call (including nil) -- optional
arguments in `cl-defun' support such a feature.

Michael.