RE: mode-line-format - local variableness

[Drew Adams]

Summary (with no understanding of why, beyond the catechismic
don't-use-let-use-setq):

This will not work:

(defun foo (text &optional buffer)
  (with-current-buffer (or buffer (current-buffer))
    (let ((mode-line-format  text))
      (force-mode-line-update) (sit-for 2))
    (force-mode-line-update)))

This works (ugh!):

(defun foo (text &optional buffer)
  (with-current-buffer (or buffer (current-buffer))
    (let ((old-mode-line-format  mode-line-format))
      (setq mode-line-format  text)
      (force-mode-line-update) (sit-for 2)
      (setq mode-line-format  old-mode-line-format))
    (force-mode-line-update)))

Don't `let'-bind `mode-line-format', but you can `let'-bind something to its
value and then restore that later. Ugh!