I am looking for something that could be used like this:

(when (cl-applicable-method-p 'foo "arg") ...)

I vaguely remember that I have seen that once and that it came with a comment saying that one shouldn't use it. However I would like to at least investigate this approach, which I cannot do if I cannot guess the name and am unable to find the implementation. (But please let me know what is wrong with using such a predicate, if anything.)

I cannot just call the generic function and prepare to handle cl-no-applicable-method errors or use a no-op default-method because I have to do some additional work before and after calling the generic function, but only if it actually is implemented given the arguments I intend to call it with

Actually the "argument" is the current major-mode, the arglist of the methods is (&context (major-mode MODE)), which probably complicates this further.

It would be nice if I could do something like:

(when (cl-applicable-method-p 'foo &context major-mode)

Another approach would be:

(defmacro foo-common (&rest body)
  `(progn (prepare)

(cl-defgeneric foo ()
  nil) ; noop

(cl-defmethod foo (&context (major-mode emacs-lisp-mode))

but that feels hackish, it would be nicer if foo-common could be replaced with a foo :around method.

I am trying to replace the various magit-...-refresh-buffer functions used in magit-refresh-buffer with a generic function.

  • I think the closest I can think of is cl-next-method-p, but I'd indeed recommend to stay away from it. Maybe we could suggest a better solution if you explained what you're hoping to gain from the use of a generic-function for magit-refresh-buffer? – Stefan Mar 14 '19 at 14:51
  • I don't particularly like (funcall (intern (concat ... "-refresh-buffer"))) and I hope for other small benefits. Nothing major, just consistency with other changes where using a generic function poses no problems. – tarsius Mar 14 '19 at 15:16
  • What I am mostly interested in is to learn why I shouldn't do this. Is it un-clos-y to ask "does this exist?" instead of calling the function and see what happens, or does the reason have more to do with implementation details? – tarsius Mar 14 '19 at 15:18
  • One way to look at the problem is that the code that signals "no such method" is basically itself a default method, and if you check for existence of a method you might find a "spurious" method (e.g. there's a :before method but no primary method, or there's a dummy default method which just signals some other error). In addition, I actively discourage use of cl-next-method-p because its implementation is horribly hackish and brittle (but implementing it in a robust way would be a lot more expensive for a feature that's basically never used but would cost even when you don't use it). – Stefan Mar 14 '19 at 22:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.