I think what Nicolas suggests is probably the easiest path. Here is one
example that does what I think you are looking for. I use a simple string
comparison on the contents, you could do something more sophisticated.

#+BEGIN_SRC emacs-lisp
(defun fruit-link-face (path)
  (let* ((ln (org-element-context))
(start (org-element-property :contents-begin ln))
(end (org-element-property :contents-end ln))
(contents (if (and start end)
      (buffer-substring start end)
    nil)))
    (if (and contents (stringp contents))
(if (string> contents "j")
   '(:foreground "red")
 '(:foreground "blue"))
      'org-link)))


(org-link-set-parameters "fruit"
                         :face 'fruit-link-face)
#+END_SRC

#+RESULTS:
| :face | fruit-link-face |

[[fruit:mango ][test]].  # this will be red

  fruit:apple. # regular org link.

 [[fruit:apple][bera]]. # this will be blue
John

-----------------------------------
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu



On Tue, Apr 21, 2020 at 1:57 PM Nicolas Goaziou <mail@nicolasgoaziou.fr>
wrote:

> Hello,
>
> Benjamin Andresen writes:
> >
> > 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.
>
> Isn't the function called with point on the link? You may just need to
> extract the contents from the environment.
>
> Otherwise, a solution is to catch `wrong-number-of-arguments' error and
> call again the function with the old calling convention. See, e.g.,
> `org-link-open'.
>
> > 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.
>
> Beware the over-engineering. At this point, someone motivated enough can
> put an advice to the link fontification function.
>
> Regards,
>
> --
> Nicolas Goaziou
>
>