5.1 Simple tracing

This section shows you how to perform simple traces.

1. Enter 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 entering the following into the listener.

   (trace fac)
   

3. Call the function fac as follows:

   (fac 3)
   

   The following trace output appears in the listener.

   0 FAC > ...
     >> N : 3
     1 FAC > ...
       >> N : 2
       2 FAC > ...
         >> N : 1
       2 FAC < ...
         << VALUE-0 : 1
     1 FAC < ...
       << VALUE-0 : 2
   0 FAC < ...
     << VALUE-0 : 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.
• The traced function name.
• The arguments and their values for the current call.

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

> denotes entry to a function, and >> denotes an argument.

Upon exit from each traced function call, trace prints the following information:

• The level of tracing.
• The traced function name.
• The returned values for the current call.

< denotes exit from a function, and << denotes a returned value.

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.

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 the function name. All tracing can be removed by calling untrace with no arguments.

CL-USER 5 > (untrace fac)
(FAC)
 
CL-USER 6 > (fac 4)
24
 
CL-USER 7 >