Locating file via load-path

[Christopher Dimech <dimech@gmx.com>]

> Sent: Saturday, August 10, 2024 at 4:19 AM
> From: "Drew Adams" <drew.adams@oracle.com>
> To: "Eli Zaretskii" <eliz@gnu.org>, "help-gnu-emacs@gnu.org" <help-gnu-emacs@gnu.org>
> Subject: RE: [External] : Re: Locating file via load-path
>
> > > > Why cl-pushnew ?
> > >
> > > Either is OK.  The doc of `add-to-list' says:
> > >
> > >   you are usually better off using `push' or `cl-pushnew'.
> > 
> > Talk about citation out of context.  The full quotation is
> > 
> >   This is handy to add some elements to configuration variables,
> >   but please do not abuse it in Elisp code, where you are usually
> >   better off using ‘push’ or ‘cl-pushnew’.
> > 
> > IOW, don't (ab)use add-to-list if all you need is to add a new element
> > to some list of elements.  add-to-list is for adding path elements to
> > variables like load-path.
> 
> That text says use it mainly for user ("configuration")
> variables, not generally for Lisp code.  And it neither
> says nor suggests anything about lists of path elements.
> 
> > > It doesn't say why, and neither does the Elisp manual.
> > 
> > Oh, yes, it does, see above.
> 
> No, there's nothing about "why" above.
> 
> May I humbly encourage you to review the following
> mails in this very list from Stefan Monnier on the
> subject, back in 2013, during his tenure as the
> Emacs maintainer (thread "Is add-to-list supposed
> to work when lexical-binding is t?"):

In such case add-to-list has to the potential for misleading code.
There’s an argument to be made that functions like add-to-list 
should be robust enough to handle a variety of scenarios, not just
for modifying configuration variables like load-path.  There should 
be some robustness expectations rather than expecting users not to 
abuse it.  The function should be versatile and resistant to misuse.

If developers could use more appropriate list operations when dealing 
with general-purpose list manipulation (including for load-path), the
better life would be for everybody. 

> Not in chronological order, but perhaps in order of
> relevance and understanding for us, here:
> 
> https://mail.gnu.org/archive/html/help-gnu-emacs/2013-06/msg00096.html
> 
> https://mail.gnu.org/archive/html/help-gnu-emacs/2013-06/msg00064.html
> 
> https://mail.gnu.org/archive/html/help-gnu-emacs/2013-06/msg00124.html
> 
> https://mail.gnu.org/archive/html/help-gnu-emacs/2013-06/msg00188.html
> 
> In particular, this bit summarizes _WHY_ he added that
> "usually better off using ‘push’ or ‘cl-pushnew’" [in
> Lisp code] text to the doc string (in Emacs 24):
> 
>   "The warning was added because add-to-list is a
>    frequent use-case where Elisp code presumes
>    dynamic binding."
> 
> And:
> 
>   > Unless you're knee-deep in the internals of the language,
>   > the difference between `cl-pushnew' and `add-to-list' is
>   > so obscure as to be incomprehensible.
> 
>   "Agreed.  I never liked add-to-list, and now I really
>    know why ... other than by ad-hoc handling the case
>    where the first arg is passed via quoting, the
>    byte-compiler" [is pretty hopeless for add-to-list]
> 
>