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9.2.24 *debugger-hook* [Variable]

Value Type::

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.

Initial Value::

nil.

Description::

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.

Examples::
      (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
Affected By::

invoke-debugger

Notes::

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.