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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.

The debug function is described in detail in The Lisp Debugger.