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11.2 Trace

When a function is traced, it means that every call to that function is reported to the user during a program run. This can help a programmer determine whether a function is being called at the wrong time or with the wrong set of arguments.

trace function

Function

Enable debug tracing on function. While a program is being run, Guile will print a brief report at each call to a traced function, advising the user which function was called and the arguments that were passed to it.

untrace function

Function

Disable debug tracing for function.

Example:

(define (rev ls)
  (if (null? ls)
      '()
      (append (rev (cdr ls))
              (cons (car ls) '())))) => rev

(trace rev) => (rev)

(rev '(a b c d e))
=> [rev (a b c d e)]
   |  [rev (b c d e)]
   |  |  [rev (c d e)]
   |  |  |  [rev (d e)]
   |  |  |  |  [rev (e)]
   |  |  |  |  |  [rev ()]
   |  |  |  |  |  ()
   |  |  |  |  (e)
   |  |  |  (e d)
   |  |  (e d c)
   |  (e d c b)
   (e d c b a)
   (e d c b a)

Note the way Guile indents the output, illustrating the depth of execution at each function call. This can be used to demonstrate, for example, that Guile implements self-tail-recursion properly:

(define (rev ls sl)
  (if (null? ls)
      sl
      (rev (cdr ls)
           (cons (car ls) sl)))) => rev

(trace rev) => (rev)

(rev '(a b c d e) '())
=> [rev (a b c d e) ()]
   [rev (b c d e) (a)]
   [rev (c d e) (b a)]
   [rev (d e) (c b a)]
   [rev (e) (d c b a)]
   [rev () (e d c b a)]
   (e d c b a)
   (e d c b a)

Since the tail call is effectively optimized to a goto statement, there is no need for Guile to create a new stack frame for each iteration. Using trace here helps us see why this is so.