How to debug elisp?

Question

Normally the easiest way to debug is using printf. What can I do to debug emacs-lisp? How can I print something to emacs editor from elisp? Or is there any way to debug elisp code?

For example, how can I check if the following code is run in .emacs file?

(load "auctex.el" nil t t)

Answer 1

The easiest way to debug may be to run your code interactively. You can do that in a lisp buffer by placing your point after the expression and running C-x C-e (eval-last-sexp).

Alternatively:

(message "hello world")

C-h f message to find out more about the built in message function. If you generate lots of messages, you may want to customize the variable message-log-max to a larger value.

Answer 2

The debugger (edebug) is pretty straight forward to use. Go to the definition of the function, and type M-x edebug-defun. The next time it is called, you'll be able to step through the code as with every other debugger. Type ? for the list of keybindings, or check out the documentation for edebug.

Answer 3

To answer your questions one by one:

• print something: there's a million ways. (message "Hello") puts the string in the echo area; (insert "hello") puts the string into the current buffer at point ...
• how can I check if the following code is run: I'd just replace "auctex.el" with (say) "frotzumotzulous" (i.e., any string at all, as long as it doesn't name a real file) and then see if you get an error message. If you get no error, then clearly that code isn't being run.