Elisp nested function stops execution

Question

I can not figure out why does when I factored out function it started to work wrong. Here's the my function to kill all dired buffers, its bound to dired-mode-map that work as expected and successfully kill all dired buffers when I invoke it in dired buffer:

(defun sandric/dired-kill-buffers ()
  (interactive)
  (mapc (lambda (buffer)
          (when (eq 'dired-mode (buffer-local-value 'major-mode buffer))
            (kill-buffer buffer)))
        (buffer-list)))

I decided to factor out a function to kill all buffers by major mode name:

(defun sandric/kill-buffers-by-major-mode (mode-name)
  "Kill open buffers by major mode name"
  (interactive)
  (mapc (lambda (buffer)
          (when (eq mode-name (buffer-local-value 'major-mode buffer))
            (kill-buffer buffer)))
        (buffer-list)))

(defun sandric/dired-kill-buffers ()
  (interactive)
  (sandric/kill-buffers-by-major-mode 'dired-mode))

And now it works wrongly - it kills only current dired buffer and not killing parent ones. I can not understand why - it looks like nested function get bounded to current buffer and stops executing after it gets killed. Can anyone explain me why is it so and how to fix it?

Answer 1

The variable mode-name is a buffer-local variable that is also bound inside your function sandric/kill-buffers-by-major-mode. Under dynamic scoping, that means that the value of mode-name seen by your code will change when you change buffers (in this case, when you kill the current buffer). See the warning here about let bindings; it also applies to function arguments.

This problem goes away for me if the function is defined using lexical scope, but according to the manual, this is undefined behavior and using a special variable such as mode-name as a function argument is discouraged.

So the correct solution is just to change the name of your function argument from mode-name to another symbol that is not defined as buffer-local.

(Thanks to Tobias for explaining the details of the problematic interaction in the comments; my previous explanation was very sketchy, and I made some unfounded assumptions about what was causing the problem.)