How to pass current-prefix-arg to this-command - search

Question

I want to reuse current-prefix-arg when calling this-command. I can imagine ways of capturing its value and passing it to modified functions. But, this doesn't seem like a great solution. How can I reuse it (eg. push it back onto a sort of call stack, or whatever emacs uses)?
The example is a simple function to increment/decrement numbers in region I was just writing where I want to temporarily bind i to increment or decrement if it was called with prefix. Can I do this without adding another argument to the function, but just using the original prefix?
(defun my-toggle-increment-numbers (start end)
"Simple function to increment numbers in region. Decrement with prefix."
(interactive "r")
(let (deactivate-mark)
(goto-char start)
(while (re-search-forward "\\([[:digit:]]+\\)" end 'move)
(replace-match (number-to-string
(+ (if current-prefix-arg -1 1)
(string-to-number (match-string 1))))
nil nil nil 1))
;; what is good way to reused the current-prefix-argument value when
;; calling this-command?
(set-transient-map
(let ((km (make-sparse-keymap)))
(define-key km "i" this-command)
km)
t)))

Answer 1

Try this:
(defun my-toggle-increment-numbers (start end &optional decrement)
"Simple function to increment numbers in region. Decrement with prefix."
(interactive "r\nP")
(let (deactivate-mark)
(setq start (copy-marker start)
end (copy-marker end))
(goto-char start)
(while (re-search-forward "\\([-]?[[:digit:]]+\\)" end 'move)
(replace-match (number-to-string (+ (if decrement -1 1)
(string-to-number (match-string 1))))
nil nil nil 1))
(set-transient-map
(let ((km (make-sparse-keymap)))
(define-key km "i" `(lambda ()
(interactive)
(my-toggle-increment-numbers ',start ',end ',decrement)))
km)
t)))
You forgot to include the minus sign preceding your [[:digit:]].
You need to either use lexical binding or construct a command that binds the current values of start, end and current-prefix-arg (or decrement - see next).
It's a bit simpler if you use an argument instead of current-prefix-arg.
(You ask if you can just use current-prefix-arg instead of adding an arg for it. Yes. In that case, let-bind current-prefix-arg in the constructed command that gets invoked for subsequent calls.)
Since replacement can change the number of digits in a numeral (e.g. from 9 to 10, you need to use a marker instead of the number value of end in subsequent calls.

Answer 2

I think the cleanest way is to create a command which calls this-command passing it the prefix arg:
;; This relies on -*- lexical-binding:t -*-
(defun my-toggle-increment-numbers (start end)
[...]
(set-transient-map
(let ((km (make-sparse-keymap))
(tc this-command)
(cpa current-prefix-arg))
(define-key km "i"
(lambda () (interactive)
(let ((current-prefix-arg cpa))
(call-interactively tc))))
km)
t)))
It's probably more verbose than what you were imagining, but I think it's the closest there is to what you're asking. A simpler way to get almost the same result could be:
(defun my-toggle-increment-numbers (start end)
[...]
(setq prefix-arg current-prefix-arg)
(set-transient-map
(let ((km (make-sparse-keymap)))
(define-key km "i" this-command)
km)
t)))