Re: Always-true predicate?

[chad <yandros@gmail.com>]

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On Mon, Feb 22, 2021 at 7:21 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: chad <yandros@gmail.com>
> > Date: Sun, 21 Feb 2021 21:02:43 -0800
> > Cc: Robert Pluim <rpluim@gmail.com>, Lars Ingebrigtsen <larsi@gnus.org>,
> Richard Stallman <rms@gnu.org>,
> >       EMACS development team <emacs-devel@gnu.org>
> >
> > I suspect that I ran into an instance of this recently, where the
> helpful package (an extension that offers
> > expanded help functions, basically) had trouble in it's helpful-callable
> function with facemenu-face-menu. I
> > tracked the problem down to a bad interaction in helpful with this code
> in facemenu.el:
> >
> >  (defvar facemenu-face-menu
> >    [...])
> >  (defalias 'facemenu-face-menu facemenu-face-menu)
> >
> > The problem that I get is:
> >
> >  Debugger entered--Lisp error: (wrong-type-argument sequencep t)
> >    mapconcat(identity (t) " ")
> >    s-join(" " (t))
> >    helpful--signature(facemenu-face-menu)
> >    helpful-update()
> >    helpful-callable(facemenu-face-menu)
> >    funcall-interactively(helpful-callable facemenu-face-menu)
> >    call-interactively(helpful-callable nil nil)
> >    command-execute(helpful-callable)
> >
> > I'm not familiar with the (defalias 'foo foo) idiom, so maybe it should
> be expected to work, and it's just a bug
> > in helpful. (I reported it to the package maintainers already, with a
> note that I'd mention it here.) There are a
> > few other instances of in emacs that I found with a quick search, and
> they also cause similar trouble for
> > helpful.
> >
> > Is this an example of an accidental functional value, as I originally
> expected, or am I barking up the wrong
> > tree and instead just looking at a parsing bug? (My lisp is largely self
> taught and started with Scheme, so
> > these parts of elisp are murky to me.)
>
> I think it's a bug in helpful: facemenu-face-menu is a keymap, so
> maybe helpful isn't ready for that.
>

Good point; I didn't (but should have) say: the issue, I think, is that the
use of defalias makes helpful-callable think that facemenu-face-menu is a
"viable callable", when it isn't. This doesn't mean that it's not a bug in
helpful, of course; I'm just trying to understand if the bug is that
helpful should be able to tell that facemenu-face-menu isn't "callable".
Here's the start of helpful-callable, fwiw:

(defun helpful-callable (symbol)
>   "Show help for function, macro or special form named SYMBOL.
>


See also `helpful-macro', `helpful-function' and `helpful-command'."
>   (interactive
>    (list (helpful--read-symbol
>           "Callable: "
>           (helpful--callable-at-point)
>           #'fboundp)))


helpful--read-symbol seems to be a simple porcelain around completing-read
with, in this case, #'fboundp as the predicate. That said, I'm not sure if
this is just a bug in helpful being confused by the idiom with defalias,
via the function-value usage you mentioned upthread.

Thanks,
~Chad

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