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Programming in Emacs Lisp

17.3 debug-on-quit and (debug)

In addition to setting debug-on-error or calling debug-on-entry, there are two other ways to start debug.

You can start debug whenever you type C-g (keyboard-quit) by setting the variable debug-on-quit to t. This is useful for debugging infinite loops.

Or, you can insert a line that says (debug) into your code where you want the debugger to start, like this:

(defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (debug) ; Start debugger. (setq number (1= number))) ; Error here. total))

The debug function is described in detail in section `The Lisp Debugger' in The GNU Emacs Lisp Reference Manual.