Re: Rebind key and save/reuse previous bound function

[John Mastro]

Joe Riel <joer@san.rr.com> wrote:
>
> Is there a clean way save the function bound to a key
> so that a new function can be created that is bound
> to the same key, does something, then executes the
> original bound function?

I don't think there's anything built-in that does exactly what you
describe, but there are a few alternatives for doing it and all are
pretty simple.

Something like this would probably be the most direct translation of
your description:

    (defun define-before (map key cmd)
      (let ((old (lookup-key map key)))
        (define-key map key
          (if (null old)
              cmd
            (lambda ()
              (interactive)
              (call-interactively cmd)
              (call-interactively old))))))

    (define-before global-map (kbd "C-n")
      (lambda ()
        (interactive)
        (message "Going to the next line!")))

However, I think using "before advice" is a nicer solution for this,
because it integrates well with `describe-function'.

    (defun my-next-line-advice (&rest args)
      (message "Going to the next line!"))

    (advice-add 'next-line :before #'my-next-line-advice)

The arguments your advice function receives will be the same as those
for the original function you're advicing, if any. Check the
documentation for `add-function' for a description of all the different
kinds of advice and what arguments the advicing function receives.

-- 
john