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  ANSI Common Lisp   9 Conditions   9.2 Dictionary of Conditions

9.2.23 break Function

Syntax:
break &optional format-control &rest format-arguments    nil

Arguments and Values:
format-control - a format control. The default is implementation-dependent.

format-arguments - format arguments for the format-control.

Description:
break formats format-control and format-arguments and then goes directly into the debugger without allowing any possibility of interception by programmed error-handling facilities.

If the continue restart is used while in the debugger, break immediately returns nil without taking any unusual recovery action.

break binds *debugger-hook* to nil  before attempting to enter the debugger.

Examples:
 (break "You got here with arguments: ~:S." '(FOO 37 A))
 BREAK: You got here with these arguments: FOO, 37, A.
 To continue, type :CONTINUE followed by an option number:
 1: Return from BREAK.
 2: Top level.
 Debug> :CONTINUE 1
 Return from BREAK.
  NIL
 

Side Effects:
The debugger is entered.

Affected By:
*debug-io*.

See Also:
error, invoke-debugger.

Notes:
break is used as a way of inserting temporary debugging "breakpoints" in a program, not as a way of signaling errors. For this reason, break does not take the continue-format-control argument that cerror takes. This and the lack of any possibility of interception by condition handling are the only program-visible differences between break and cerror.

The user interface aspects of break and cerror are permitted to vary more widely, in order to accomodate the interface needs of the implementation. For example, it is permissible for a Lisp read-eval-print loop to be entered by break rather than the conventional debugger.

break could be defined by:

 (defun break (&optional (format-control "Break") &rest format-arguments)
   (with-simple-restart (continue "Return from BREAK.")
     (let ((*debugger-hook* nil))
       (invoke-debugger
           (make-condition 'simple-condition
                           :format-control format-control
                           :format-arguments format-arguments))))
   nil)

Allegro CL Implementation Details:
None.

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