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~
(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.
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