Global minor mode not global?

Question

I am trying to write a global minor mode. The following illustrates my problem:

(define-minor-mode global-foo-mode :global t :lighter " FOO")

then run

M-x global-foo-mode

The message displayed is

Global-Foo mode disabled

The current buffer does not display the FOO lighter in its mode line, but every other buffer does. Subsequent invocations of M-x global-foo-mode in various buffers seem to enable and disable the mode on the current buffer, but not globally as I would expect. Can anyone help me understand what I'm doing wrong here?

Answer 1

The problem is that you are leaving out the required argument DOC when you call define-minor-mode. (Or more accurately, you are passing :global as the DOC parameter)

Either provide a doc string or nil as the second argument:

(define-minor-mode global-foo-mode nil :global t :lighter " FOO")

Note that define-minor-mode is a macro. You can see how it is expanded. Put point just after the closing paren and call M-x pp-macroexpand-last-sexp. Try this with and without the nil in there, and you'll see that in your case :global is being used where the docstring should be.