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3.10.2 Tracing Examples

Here is an example of tracing with some of the possible options. For simplicity, this is the function: 

    (defun fact (n)
      (declare (double-float n) (optimize speed))
      (if (zerop n)
          1d0
          (* n (fact (1- n)))))
    (compile 'fact)

This example shows how to use the :condition option: 

    (trace fact :condition (= 4d0 (debug:arg 0)))
    (fact 10d0) ->
      0: (FACT 4.0d0)
      0: FACT returned 24.0d0
    3628800.0d0

As we can see, we produced output when the condition was satisfied.

Here’s another example: 

    (untrace)
    (trace fact :break (= 4d0 (debug:arg 0)))
    (fact 10d0) ->
      0: (FACT 5.0d0)
        1: (FACT 4.0d0)


    Breaking before traced call to FACT:
       [Condition of type SIMPLE-CONDITION]

    Restarts:
      0: [CONTINUE] Return from BREAK.
      1: [ABORT   ] Return to Top-Level.

    Debug  (type H for help)

In this example, we see that normal tracing occurs until we the argument reaches 4d0, at which point, we break into the debugger.