Allows arbitrary functions to become the standard Lisp debugger

Upstream URL


Michał "phoe" Herda <>





This is a portability library that allows one to fully override the standard debugger provided by their Common Lisp system for situations where binding *debugger-hook* is not enough - most notably, for #'break.

  • The main interface is the with-debugger macro that accepts a hook function that would be acceptable for *debugger-hook* (it must accept two arguments: a condition that the debugger is invoked with and a similar hook function).
  • A functional interface for the above is provided in form of call-with-debugger, which accepts a hook function and a zero-argument thunk.
  • It is possible, though not advisable, to install a debugger function globally across the whole Lisp system. The function install-debugger is provided for that case. (If you want to use it, you might want to contribute a matching uninstall-debugger function that restores the Lisp system to the previous state.)

Inside the debugger function:

  • the value of *debugger-hook* is nil,
  • the value of the second argument passed to the debugger function is the debugger function itself.


A basic building block that allows one to implement a portable, custom debugger for Common Lisp systems in place of the original system-provided one.


TRIVIAL-CUSTOM-DEBUGGER> (with-debugger ((lambda (condition hook)
                                           (declare (ignore hook))
                                           (format t ";; Debugging a ~S!~%" 
                                                   (type-of condition))
                                           (throw :handled t)))
                           (list (catch :handled (error 'error))
                                 (catch :handled (break))
                                 (let ((*break-on-signals* 'error))
                                   (catch :handled (signal 'error)))
                                 (catch :handled 
                                   (invoke-debugger (make-condition 'error)))))
;; Debugging a ERROR!
;; Debugging a SIMPLE-CONDITION!
;; Debugging a SIMPLE-CONDITION!
;; Debugging a ERROR!
(T T T T)


Inside with-debugger or after calling install:

  • the value of *debugger-hook* is undefined,
  • the consequences are undefined if *debugger-hook* is set or rebound,
  • it is undefined whether the debugger hook or the debugger will be invoked first


Very roughly tested on SBCL, CCL, ECL, Clasp, ABCL, CLISP, ACL, LispWorks, and Mezzano.

To run the test suite, evaluate (asdf:test-system :trivial-custom-debugger).

Dependencies (1)

  • parachute
  • GitHub
  • Quicklisp