Delete trailing whitespace when saving, except certain modes

Question

How can I delete-trailing-whitespace when saving, except for certain modes? With this attempt, whitespace is deleted for all modes, including fundamental-mode, which I'm trying to avoid.

(add-hook 'before-save-hook
             (when '(not fundamental-mode))
               'delete-trailing-whitespace)

Answer 1

(add-hook 'before-save-hook
          (when '(not fundamental-mode))
             'delete-trailing-whitespace)

You are not passing a function as the second arg to add-hook. You are passing the symbol delete-trailing-whitespace, systematically.

add-hook is a function, so all of its args are evaluated before it does its thing (using their values). The value of the sexp you passed as its second arg is the symbol delete-trailing-whitespace, because the arg to when is a non-nil list (because you quoted it). So the when condition is always true.

And you never invoke function delete-trailing-whitespace; you just return the symbol that names that function.

You need to pass a function to add-hook. Something like this:

(add-hook 'before-save-hook
          (lambda ()
            (unless (eq major-mode 'fundamental-mode)
              (delete-trailing-whitespace))))

But are you sure that you want to do that for every possible mode other than fundamental-mode? If not, consider checking major-mode against a whitelist or a blacklist of modes.

And consider using derived-mode-p to test modes - it is typically more appropriate than testing them using eq.