4 The Trace Facility

4.1 Simple tracing

This section shows you how to perform simple traces.

: 1. Type this definition of the factorial functionfac 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 functionfac as follows:

(fac 3)

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 totrace (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.

Callingtrace with no arguments produces a list of all the functions currently being traced. In order to cease tracing a function the macrountrace should be called with commands. All tracing can be removed by callinguntrace with no arguments.

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

CL-USER 7 >