4.1 Simple tracing

This section shows you how to perform simple traces.

1. Type this definition of the factorial function fac into the listener:

(defun fac (n)

 (if (= n 1) 1

   (* n (fac (- n 1)))))

2. Now trace the function by typing the following into the listener.
(trace fac)
3. Call the function fac as follows:

The following trace output appears in the listener.

0 FAC > (3)

  1 FAC > (2)

    2 FAC > (1)

    2 FAC < (1)

  1 FAC < (2)

0 FAC < (6)

Upon entry to each traced function call, trace prints the following information:

• The level of tracing, that is, the number of recursive entries to trace (starting at 0).
• The function name.
• The argument for the current call.

Each call is indented according to the level of tracing for the call.

Upon exit from each call, the same information is produced: The > symbol denotes entry to a function, and the < symbol denotes exit from it.

Output produced in this way is always sent to a special stream, *trace-output* , which is either associated with the listener, or with background output. You can give other expressions to be sent to this stream, in addition to the arguments and results of a function.

Calling trace with no arguments produces a list of all the functions currently being traced. In order to cease tracing a function the macro untrace should be called with commands. All tracing can be removed by calling untrace with no arguments.

CL-USER 5 > (untrace fac)

NIL

CL-USER 6 > (fac 4)

24

CL-USER 7 >