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  ANSI Common Lisp   9 Conditions   9.2 Dictionary of Conditions

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~%" 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

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.

Allegro CL Implementation Details:
None.

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