Re: [Emacs-diffs] scratch/so-long 7273fb2: Add so-long library

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

>>     (advice-add 'hack-local-variables :around #'so-long--hlv)
>>     (defun so-long--hlv (orig-fun &optional handle-mode &rest args)
>>       ;; The first arg to `hack-local-variables' is HANDLE-MODE since
> Emacs 26.1,
>>       ;; and MODE-ONLY in earlier versions.  In either case we are
> interested in
>>       ;; whether it has the value `t'.
>>       (let ((retval (apply orig-fun handle-mode args)))
>>         (and (eq handle-mode t)
>>              retval ; A file-local mode was set.
>>              (so-long-handle-file-local-mode retval))))
>
> This is changing 'after' advice into 'around' advice, which means that
> your version changes the return value, so that's definitely not
> equivalent.

Oops, indeed, sorry.

> I've changed it to explicitly return retval.

Good, thanks.

> Can nadvice not do 'after' advice which knows the arguments?

The arguments, yes, but the just-computed return value, no.

> The "Files with a file-local 'mode'" section of the Commentary says
> much the same, but I'd not yet added that at the time you queried
> this.  I think that ought to provide the missing context for readers
> (as otherwise I think all the advice is documented pretty clearly?)

I must admit that I do a lot of hacking on packages I never use, so
I like it when the code explains what it does without requiring the
coder to have read the doc ;-)

>>> Such changes will need to work in Emacs 25 at least, and ideally
>>> back to 24.3
>> advice-add was added to Emacs-24.4 and it is available in GNU ELPA
>> for earlier Emacsen, so it's OK.
> So: Package-Requires: ((emacs "24.3") nadvice), yes?
> No -- experimentally that causes Emacs 26.1 to install nadvice from
> ELPA instead of deferring to the library it already has.

IIUC that's a bug in Emacs-26.1 (and 26.2) because 26.[12] is not aware
that it comes with nadvice-1.0.

It really should have been fixed before Emacs-26.2, but I somehow forgot
to push that commit and it lingered on a machine I only used again
very recently :-(


        Stefan