Cannot set global variable with dolist in Elisp

Question

I want to exclude modes that don't need show-trailing-whitespace:

(set-default 'show-trailing-whitespace t)

(defmacro non-trailing-whitespaces-modes (mode)
  (let ((hook (intern (concat  (symbol-name mode) "-mode-hook"))))
    `(add-hook ',hook (lambda () (setq show-trailing-whitespace nil)))))

(defvar trailing-whitespace-exclude-modes '(eshell term help diff ztree org calendar syslog slime-repl Info ielm))

(dolist (m trailing-whitespace-exclude-modes)
  (non-trailing-whitespaces-modes m))

However, I cannot add hook using dolist, but adding idividual call for each mode works, for example:

(non-trailing-whitespaces-modes eshell)
(non-trailing-whitespaces-modes term)
(non-trailing-whitespaces-modes ielm

What's wrong?

Answer 1

Simple:

(macroexpand '(non-trailing-whitespaces-modes m))
;; (add-hook (quote m-mode-hook)
;;           (lambda nil (setq show-trailing-whitespace nil)))

Your task does not really require you to use macros. Turn your macro into a function and you're done.

Here's the equivalent defun that can be used in dolist:

(defun non-trailing-whitespaces-modes (mode)
  (let ((hook (intern (concat (symbol-name mode) "-mode-hook"))))
    (add-hook hook (lambda () (setq show-trailing-whitespace nil)))))