a designator for a function of two arguments (a condition and the value of *debugger-hook* at the time the debugger was entered), or nil.
When the value of *debugger-hook* is non-nil, it is called prior to normal entry into the debugger, either due to a call to invoke-debugger or due to automatic entry into the debugger from a call to error or cerror with a condition that is not handled. The function may either handle the condition (transfer control) or return normally (allowing the standard debugger to run). To minimize recursive errors while debugging, *debugger-hook* is bound to nil by invoke-debugger prior to calling the function.
(defun one-of (choices &optional (prompt "Choice")) (let ((n (length choices)) (i)) (do ((c choices (cdr c)) (i 1 (+ i 1))) ((null c)) (format t "~&[~D] ~A~%" i (car c))) (do () ((typep i `(integer 1 ,n))) (format t "~&~A: " prompt) (setq i (read)) (fresh-line)) (nth (- i 1) choices))) (defun my-debugger (condition me-or-my-encapsulation) (format t "~&Fooey: ~A" condition) (let ((restart (one-of (compute-restarts)))) (if (not restart) (error "My debugger got an error.")) (let ((*debugger-hook* me-or-my-encapsulation)) (invoke-restart-interactively restart)))) (let ((*debugger-hook* #'my-debugger)) (+ 3 'a)) >> Fooey: The argument to +, A, is not a number. >>  Supply a replacement for A. >>  Return to Cloe Toplevel. >> Choice: 1 >> Form to evaluate and use: (+ 5 'b) >> Fooey: The argument to +, B, is not a number. >>  Supply a replacement for B. >>  Supply a replacement for A. >>  Return to Cloe Toplevel. >> Choice: 1 >> Form to evaluate and use: 1 => 9
See Also: None.
When evaluating code typed in by the user interactively, it is sometimes useful to have the hook function bind *debugger-hook* to the function that was its second argument so that recursive errors can be handled using the same interactive facility.