Sometimes in emacs nil is not punned well enough. I found this when using Telephone Line, when the crucial problem is that nil can’t be construed as a string (why isn’t it the empty string when strings are wanted?).

Error during redisplay: (eval (telephone-line-add-subseparators ‘((lambda (face) (telephone-line-raw (if (telephone-line-selected-window-active) (progn (propertize battery-mode-line-string ‘mouse-face ‘(:box 1))))))) telephone-line-cubed-hollow-left ‘evil)) signaled (wrong-type-argument stringp nil) [36 times]

I fixed this manually with some extra code, when writing such cruft feels like a slight against lisps:

(telephone-line-defsegment telephone-line-battery-segment ()
  (if (and (telephone-line-selected-window-active)
           battery-mode-line-string)
    (propertize battery-mode-line-string
                'mouse-face '(:box 1))
    ""))

Notice the value of battery-mode-line-string may be nil, like when I have a limit to the percentages at which it should display, and this was causing lots of errors about (stringp nil) that cluttered my error messages on every buffer change.

Resources