how to supply universal-argument (C-u) to a function call inside a binding statement [duplicate]

Question

I have this on my init.el

;; magit status
(define-key evil-motion-state-map "gs" 'magit-status)

But I would like to have another binding, for C-u M-x magit-status (C-u is the universal-argument)

Something like (which obviously does not work):

;; magit status
(define-key evil-motion-state-map "ga" 'C-u magit-status)

rationale: with C-u M-x magit-status, magit allows you to supply a directory where a git repo resides, so you do not have to have an opened file that resides inside the git repo where you want to perform git actions.

Answer 1

#'magit-status is fun, because its argument to interactive isn't just a string, so running it naively with call-interactively doesn't work.Instead, the function generates its own interactive arguments. Here's the interactive call inside (defun magit-status ...:

(interactive
 (list (and (or current-prefix-arg (not (magit-toplevel)))
            (magit-read-repository
             (>= (prefix-numeric-value current-prefix-arg) 16)))))

So we have to bind current-prefix-arg directly, to pretend it's been called with a prefix arg:

(define-key evil-motion-state-map "ga"
 (lambda () (interactive)
   (let ((current-prefix-arg 4))
     (call-interactively #'magit-status))))

Or, as I tested it (because I don't use evil-mode):

(global-set-key
 (kbd "C-c C-s")
 (lambda () (interactive)
   (let ((current-prefix-arg 4))
     (call-interactively #'magit-status))))

Thanks to @npostavs for the instruction on binding current-prefix-arg. The previous answer (for the purposes of transparency) was:

(global-set-key
 (kbd "C-c C-s")
 (lambda () (interactive)
   (magit-status (magit-read-repository
                  (>= (prefix-numeric-value current-prefix-arg) 16)))))

Answer 2

You do not need a separate keybinding to pass the universal argument to the command. Based on your current keybinding, you may simply type C-ugs.

Answer 3

Even better, if you don't want to risk doing the wrong thing when/if magit-status is changed:

(define-key evil-motion-state-map "ga"
  (lambda () (interactive)
    (universal-argument)
    (call-interactively 'magit-status))

or to test without using evil:

(global-set-key
 (kbd "C-c C-s")
 (lambda () (interactive)
   (universal-argument)
   (call-interactively 'magit-status)))

Basically universal-argument sets up the state as if you had pressed C-u (because C-u is actually bound to universal-argument), then the next interactively-called function reads that state. If you wanted to call a function as if C-u had been pressed twice, then you would first call universal-argument and then universal-argument-more. Numeric arguments could be supplied with digit-argument and negative-argument; see simple.el for the actual code that implements all of this.