How to call interactive Emacs Lisp function with a prefix argument, from another Emacs Lisp function

elispemacs

I want to write an Emacs Lisp function that will turn on flyspell-mode regardless of the current state of the mode. Function flyspell-mode-on is deprecated. The documentation suggests that a positive prefix argument will turn flyspell-mode, but unfortunately running

(flyspell-mode 1)

results in an error message:

Wrong number of arguments: (lambda (flyspell-mode 1)), 0

If I could figure out how to call flyspell-mode with a prefix argument, I believe I could solve this problem.

The most relevant section I can find in the Emacs Lisp manual is the section entitled "Interactive Call", which describes such commands as call-interactively. This is emphatically not what I want.

(The ultimate problem I am trying to solve is to create a mode hook that turns on the mode regardless of its current state.)

N.B. The title of the question emacs lisp call function with prefix argument programmatically makes it appear to be related, but that question was asking about how to create an interactive command, and the issue was ultimately resolved by using call-interactively.

EDIT: This question is moot; I have found an alternate solution to my original problem:

(add-hook 'text-mode-hook
          (function (lambda ()
                      (require 'flyspell)
                      (if flyspell-mode nil (flyspell-mode)))))

But I would still like to know how to call an Emacs Lisp function with a prefix argument, from another Emacs Lisp function, with nothing interactive.

UPDATE: Perhaps I should have asked why I was getting that error message…

Best Answer

It looks like your version of Flyspell mode does not follow the minor mode conventions, which require that you can turn on a minor mode with (name-of-mode t) or any positive prefix argument, turn it off with (name-of-mode 0) any negative prefix argument, and toggle it with (name-of-mode nil).

If you have the latest version of Flyspell, a bug report might be in order. I have the version shipped with GNU Emacs 23.2 on my machine, and it respects the convention. My version also defines two functions turn-on-flyspell and turn-off-flyspell, both trivial wrappers around flyspell-mode; functions with such names are common, but not official conventions. The functions flyspell-mode-on and flyspell-mode-off are apparently intended for internal use.

As a general matter, commands read the current prefix argument from the current-prefix-arg variable. Don't confuse that with prefix-arg, which is the value for the next command (only a few commands like universal-argument touch this variable). Thus, if you need to pass a prefix argument when calling a function, bind or set current-prefix-arg.

(let ((current-prefix-arg t))
  (flyspell-mode))
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