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.

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. 
⊳  [1] Supply a replacement for A. 
⊳  [2] Return to Cloe Toplevel. 
⊳ Choice: 1 
⊳  Form to evaluate and use: (+ 5 'b) 
⊳  Fooey: The argument to +, B, is not a number. 
⊳  [1] Supply a replacement for B. 
⊳  [2] Supply a replacement for A. 
⊳  [3] Return to Cloe Toplevel. 
⊳ Choice: 1 
⊳  Form to evaluate and use: 1 
→ 9

invoke-debugger

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.