Public API change: How to handle function signature change gracefully

[Benjamin Andresen via "General discussions about Org-mode." <emacs-orgmode@gnu.org>]

Hello everyone,

I would like to change the public API of the :face part of `org-link-set-parameters':

        (org-link-set-parameters "file" :face 'org-link)

My ultimate goal is to have org-links be able to be have their face changed based on the contents, not just the path of the link.

I found the relevant code in org.el in the function `org-activate-links':

   'face (pcase (org-link-get-parameter type :face)
   ((and (pred functionp) face) (funcall face path))
   ((and (pred facep) face) face)
   ((and (pred consp) face) face) ;anonymous
   (_ 'org-link))

and would like to change this to 

   'face (pcase (org-link-get-parameter type :face)
   ((and (pred functionp) face) (funcall face path contents)) ;; this is the change
   ((and (pred facep) face) face)
   ((and (pred consp) face) face) ;anonymous
   (_ 'org-link))

Now that will introduce a host of call issues because the callees don't expect to the amount of arguments changed under their bottom.

I would like some guidance how I could get what I think is neat: The contents of the bracket-style links as an additional parameter to set faces on and not breaking existing hookups.

I'm thinking to change the above code to look at the callee's function signature and checking how many arguments it accepts and then use the right funcall for the situation.

To make this more palatable I would suggest it's changed so that the 2nd argument will be a list of alists or keywords so to not have this function signature problem if someone else comes up with a reason to introduce yet more data.

Thanks in advance,
Benjamin Andresen