How to figure out which parameters a function actually need?

Question

I would like to insert the number 1, and then increase the number with calling the function evil-numbers/inc-at-pt from the package evil-numbers.

So I went looking into the source code of the function evil-numbers/inc-at-pt:

(defun evil-numbers/inc-at-pt (amount &optional no-region)
"Increment the number at point or after point before end-of-line by `amount'.
When region is selected, increment all numbers in the region by `amount'

NO-REGION is internal flag that allows
`evil-numbers/inc-at-point' to be called recursively when
applying the regional features of `evil-numbers/inc-at-point'.

"
(interactive "p*")
[...]

This function don't need any region by the way. You can place your cursor after the number, without any region. It will increase the number if there is no region, then it will look for any number and increase it.

So I created a function around this

(defun eshell-go-to-number-down ()
    (interactive "p*")
    (evil-insert)
    (insert "0")
    (evil-numbers/inc-at-pt 1))

So when I call that function, it will insert the number and increase it. I plan to modify it, when there is already a number then don't insert any number. But that's a later concern. I'm trying to figure out how I call evil-numbers/inc-at-pt properly.

As it seems from the source code, the function evil-numbers/inc-at-pt expects a number, which I provide with 1 But I get the error message:

funcall-interactively: Wrong number of arguments: (lambda nil (interactive "p*") (evil-insert) (insert "1") (evil-numbers/inc-at-pt 1)), 1 

So where went I wrong? Please don't verdict me, I'm trying to learn Lisp.

Answer 1

You've misunderstood the error. Emacs is complaining about the call to your function eshell-go-to-number-down; not the call to evil-numbers/inc-at-pt.

Your problem is that you have defined a function with an empty arglist (), and then given it an interactive spec of "p*" -- which supplies an argument when the function is called interactively.

See C-h f interactive if you're uncertain of what that spec means. Also C-h i g (elisp) Prefix Command Arguments. Note that the "numeric value" of a nil prefix argument is 1, which may have further confused you.

To resolve the conflict you either need to change the interactive spec, or else introduce an argument to your function.