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.