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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.

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