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Re: dynamic variables and advice

From: Stefan Monnier
Subject: Re: dynamic variables and advice
Date: Thu, 03 Mar 2022 10:03:22 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

> I was trying to use the add-function facility to make dired and friends
> prompt only for directories. the naive approach with a lot of printf
> debugging -
> #+begin_src elisp
> (defvar read-file-name--directories-p nil
>   "dynamic variable which should be let-bound in an :around advice
> and should be used in a :filter-args advice.")
> (defun read-file-name--maybe-filter-directories (args)
>   "filter-args advice for read-file-name."
>   (cl-destructuring-bind (prompt &optional dir default-filename
>                                mustmatch initial predicate)
>       args
>     (let ((ret (list prompt dir default-filename mustmatch initial
>                    (or predicate
>                        (and read-file-name--maybe-filter-directories-p
>                             #'file-directory-p)))))
>       (message "rfn-mfd: rfnmfd=%s this-command=%s ret=%s"
>              read-file-name--maybe-filter-directories-p  this-command ret)
>       ret)))
> (advice-add 'read-file-name
>           :filter-args 'read-file-name--maybe-filter-directories)
> (defun rfnmfd-yes (oldfun &rest args)
>   "an around advice function which sets rfn-mfd-p to t"
>   (let ((read-file-name--mabe-filter-directories-p t))
>     (message "rfnmfd-yes: begin")
>     (prog1 (apply oldfun args)
>       (message "rfnmfd-yes: done"))))
> (advice-add 'dired-read-dir-and-switches :around 'rfnmfd-yes)
> #+end_src
> Now when I call M-x dired the around advice for
> dired-read-dir-and-switches runs, and it binds
> read-file-name--directories-p to t and eventually goes on to call
> read-file-name where the filter-args advice kicks in.
> but in filter-args advice my dynamic variable  is bound to nil and not
> to t as I'd expect.
> How is my expectation wrong or what is actually happening?

A typo, "maybe"?
[ Pun intended.  ]


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