dynamic variables and advice

[Madhu <enometh@meer.net>]

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?