Wrong type argument: stringp, nil

Question

I am essentially trying to build a list of strings, where each of those strings in turn are built by their own functions.

However, I get the following error when the code is evaluated:

Wrong type argument: stringp, nil

Here is the code:

(defun mode-line-segment-git-status ()
  "Build the git status mode-line segment."
  (let (branch-name '(substring-no-properties vc-mode 5)))
  (list "Branch: " 'branch-name))


(defvar mode-line-segments nil)
(defun add-mode-line-segment (segment)
  "Add a modeline segment to the modeline."
  (setq mode-line-segments (append mode-line-segments (funcall segment))))

(add-mode-line-segment 'mode-line-segment-git-status)

(setq-default mode-line-format mode-line-segments)

What am I doing wrong?

Answer 1

@Dan's comment is quite correct: you don't need to be quoting inside your let form, and also, you can't access let-bound variables outside the scope of the let form. (You might notice that Emacs lisp has a single global namespace for variables. let is how we encapsulate variables inside functions, keep them from leaking.)

More generally: the Wrong type argument is elisp's way of telling you, "I expected an x but got a y. stringp, nil means, "I expected something that satisfies the stringp predicate, but I received nil." You can get more information about where the error is happening by calling M-x toggle-debug-on-error, but "I'm handing a nil to something that needs a string` might be enough of a hint.