You entered this new world of Lisp and now wonder: how can we debug what’s going on ? How is it more interactive than in other platforms ? What does bring the interactive debugger appart from stacktraces ?

note: this tutorial is available on the Common Lisp Cookbook and it will receive updates there.

If you want step-by-step examples of interactive debugging with nice screenshots and gifs, see the blog posts in the References section below.

Table of Contents

Well of course we can use the famous technique of “print debugging”. Let’s just recap a few print functions.

print works, it prints a READable representation of its argument, which means what is printed can be read back in by the Lisp reader.

princ focuses on an aesthetic representation.

format t "~a" …), with the aesthetic directive, prints a string (in t, the standard output stream) and returns nil, whereas format nil … doesn’t print anything and returns a string. With many format controls we can print several variables at once.


Logging is a good evolution from print debugging ;)

log4cl is the popular, de-facto logging library but it isn’t the only one. Download it:

(ql:quickload :log4cl)

and let’s have a dummy variable:

(defvar *foo* '(:a :b :c))

We can use log4cl with its log nickname, then it is as simple to use as:

(log:info *foo*)
;; <INFO> [13:36:49] cl-user () - *FOO*: (:A :B :C)

We can interleave strings and expressions, with or without format control strings:

(log:info "foo is " *foo*)
;; <INFO> [13:37:22] cl-user () - foo is *FOO*: (:A :B :C)
(log:info "foo is ~{~a~}" *foo*)
;; <INFO> [13:39:05] cl-user () - foo is ABC

With its companion library log4slime, we can interactively change the log level:

  • globally,
  • per package,
  • per function,
  • and by CLOS methods and CLOS hierarchy (before and after methods).

It is very handy, when we have a lot of output, to turn off the logging of functions or packages we know to work, and thus narrowing our search to the right area. We can even save this configuration and re-use it in another image, be it on another machine.

We can do all this through commands, keyboard shortcuts and also through a menu or mouse clicks.

"changing the log level with log4slime"

We invite you to read log4cl’s readme.

Using the powerful REPL

Part of the joy of Lisp is the excellent REPL. Its existence usually delays the need to use other debugging tools, if it doesn’t annihilate them for the usual routine.

As soon as we define a function, we can try it in the REPL. In Slime, compile a function with C-c C-c (the whole buffer with C-c C-k), switch to the REPL with C-c C-z and try it. Eventually enter the package you are working on with (in-package :your-package).

The feedback is immediate. There is no need to recompile everything, nor to restart any process, nor to create a main function and define command line arguments for use in the shell (we can do this later on when needed).

We usually need to create some data to test our function(s). This is a subsequent art of the REPL existence and it may be a new discipline for newcomers. A trick is to write the test data alongside your functions but inside a #+nil declaration so that only you can manually compile them:

       (defvar *test-data* nil)
       (setf *test-data* (make-instance 'foo …)))

When you load this file, *test-data* won’t exist, but you can manually create it with a C-c C-c away.

We can define tests functions like this.

Some do similarly inside #| … |# comments.

All that being said, keep in mind to write unit tests when time comes ;)

Inspect and describe

These two commands share the same goal, printing a description of an object, inspect being the interactive one.

(inspect *foo*)

The object is a proper list of length 3.
0. 0: :A
1. 1: :B

2. 2: :C
> q

We can also, in editors that support it, right-click on any object in the REPL and inspect them. We are presented a screen where we can dive deep inside the data structure and even change it.

Let’s have a quick look with a more interesting structure, an object:

(defclass foo ()
    ((a :accessor foo-a :initform '(:a :b :c))
     (b :accessor foo-b :initform :b)))
(make-instance 'foo)
;; #<FOO {100F2B6183}>

We right-click on the #<FOO object and choose “inspect”. We are presented an interactive pane (in Slime):

#<FOO {100F2B6183}>
 Group slots by inheritance [ ]
 Sort slots alphabetically  [X]

All Slots:
[ ]  A = (:A :B :C)
[ ]  B = :B

[set value]  [make unbound]

When we click or press enter on the line of slot A, we inspect it further:

#<CONS {100F5E2A07}>
A proper list:
0: :A
1: :B
2: :C

The interactive debugger

Whenever an exceptional situation happens (see error handling), the interactive debugger pops up.

It presents the error message, available actions (restarts), and the backtrace. A few remarks:

  • the restarts are programmable, we can create our owns,
  • in Slime, press v on a stacktrace to be redirected to the source file at the right line,
  • hit enter on a frame for more details,
  • we can explore the functionnality with the menu that should appear in our editor. See below in “break” section for a few more commands (eval in frame, etc).

Usually your compiler will optimize things out and this will reduce the amount of information available to the debugger. For example sometimes we can’t see intermediate variables of computations. You might want to print a function argument (with e to “eval in frame”, see below), but you keep getting a Variable XYZ is unbound error.

To fix this, we have to change the optimization choices with declaim, at the beginning of the file:

(declaim (optimize (speed 0) (space 0) (debug 3)))

or with declare, inside a defun:

(defun my-fun (xyz)
  (declare (optimize (debug 3)))

and recompile the code. Now you should be able to see local variables such asxyz.


trace allows us to see when a function was called, what arguments it received, and the value it returned.

(defun factorial (n)
  (if (plusp n)
    (* n (factorial (1- n)))
(trace factorial)

(factorial 2)
  0: (FACTORIAL 3)
    1: (FACTORIAL 2)
      2: (FACTORIAL 1)
        3: (FACTORIAL 0)
        3: FACTORIAL returned 1
      2: FACTORIAL returned 1
    1: FACTORIAL returned 2
  0: FACTORIAL returned 6

(untrace factorial)

To untrace all functions, just evaluate (untrace).

In Slime we also have the shortcut C-c M-t to trace or untrace a function.

If you don’t see recursive calls, that may be because of the compiler’s optimizations. Try this before defining the function to be traced:

(declaim (optimize (debug 3)))

The output is printed to *trace-output* (see the CLHS).

In Slime, we also have an interactive trace dialog with M-x slime-trace-dialog bound to C-c T.

Tracing method invocation

In SBCL, we can use (trace foo :methods t) to trace the execution order of method combination (before, after, around methods). For example:

(trace foo :methods t)

(foo 2.0d0)
  0: (FOO 2.0d0)
    1: ((SB-PCL::COMBINED-METHOD FOO) 2.0d0)
      2: ((METHOD FOO (FLOAT)) 2.0d0)
        3: ((METHOD FOO (T)) 2.0d0)
        3: (METHOD FOO (T)) returned 3
      2: (METHOD FOO (FLOAT)) returned 9
      2: ((METHOD FOO :AFTER (DOUBLE-FLOAT)) 2.0d0)
    1: (SB-PCL::COMBINED-METHOD FOO) returned 9
  0: FOO returned 9

See the CLOS section for a tad more information.


step is an interactive command with similar scope than trace. This:

(step (factorial 2))

gives an interactive pane with the available restarts:

Evaluating call:
With arguments:
   [Condition of type SB-EXT:STEP-FORM-CONDITION]

 0: [STEP-CONTINUE] Resume normal execution
 1: [STEP-OUT] Resume stepping after returning from this function
 2: [STEP-NEXT] Step over call
 3: [STEP-INTO] Step into call
 4: [RETRY] Retry SLIME REPL evaluation request.
 5: [*ABORT] Return to SLIME's top level.

  0: ((LAMBDA ()))

Stepping is useful, however it may be a sign that you need to simplify your function.


A call to break makes the program enter the debugger, from which we can inspect the call stack.

Breakpoints in Slime

Look at the SLDB menu, it shows navigation keys and available actions. Of which:

  • e (sldb-eval-in-frame) prompts for an expression and evaluates it in the selected frame. This is how we can explore our intermediate variables.
  • d is similar with the addition of pretty printing the result.

Once we are in a frame and detect a suspicious behavior, we can even re-compile a function at runtime and resume the program execution from where it stopped (using the “step-continue” restart).

Advise and watch

advise and watch are available in some vendor implementations, like LispWorks. They are not available in SBCL. advise allows to modify a function without changing its source, or to do something before or after its execution, like CLOS’ method combination (befor, after around methods).

watch allows to specify variables to be displayed in some GUI during the program execution.

Unit tests

Last but not least, automatic testing of functions in isolation might be what you’re looking for ! See the testing section and a list of test frameworks and libraries.