How to know whether a function is implemented in C or Elisp?

Question

I want to switch some behavior in my .emacs based on whether I have the new native fast json-serialize which is implemented in C rather than Elisp.

Is there a simple way to detect whether a function is implemented in C rather than Elisp? I assume there must be because C-h f json-serialize knows it.

Answer 1

I did C-h f C-h f and got a *Help* buffer describing describe-function as a "Lisp function" defined in help-fns.el. I clicked on the file name and searched for "Lisp function" which lead me to the function help-fns-function-description-header whose code indicated that what we want is subrp.

(subrp (symbol-function 'car))
==> nil

A bit more consideration would point to help-fns--analyze-function which disentangles things like aliasing and advising.

I think, however, that you should look at the "source" rather than the result here, i.e., check that the lisp file where json-serialize could be defined has been loaded - use featurep for that.

Answer 2

(subrp (symbol-function 'json-serialize))

should do it. See Functions for details.