guile/doc/ref/scheme-debugging.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Tracing
@section Tracing

The @code{(ice-9 debug)} module implements tracing of procedure
applications.  When a procedure is @dfn{traced}, it means that every
call to that procedure is reported to the user during a program run.
The idea is that you can mark a collection of procedures for tracing,
and Guile will subsequently print out a line of the form

@lisp
|  |  [@var{procedure} @var{args} @dots{}]
@end lisp

whenever a marked procedure is about to be applied to its arguments.
This can help a programmer determine whether a function is being called
at the wrong time or with the wrong set of arguments.

In addition, the indentation of the output is useful for demonstrating
how the traced applications are or are not tail recursive with respect
to each other.  Thus, a trace of a non-tail recursive factorial
implementation looks like this:

@lisp
[fact1 4]
|  [fact1 3]
|  |  [fact1 2]
|  |  |  [fact1 1]
|  |  |  |  [fact1 0]
|  |  |  |  1
|  |  |  1
|  |  2
|  6
24
@end lisp

While a typical tail recursive implementation would look more like this:

@lisp
[fact2 4]
[facti 1 4]
[facti 4 3]
[facti 12 2]
[facti 24 1]
[facti 24 0]
24
@end lisp

@deffn {Scheme Procedure} trace procedure
Enable tracing for @code{procedure}.  While a program is being run,
Guile will print a brief report at each call to a traced procedure,
advising the user which procedure was called and the arguments that were
passed to it.
@end deffn

@deffn {Scheme Procedure} untrace procedure
Disable tracing for @code{procedure}.
@end deffn

Here is another example:

@lisp
(define (rev ls)
  (if (null? ls)
      '()
      (append (rev (cdr ls))
              (cons (car ls) '())))) @result{} rev

(trace rev) @result{} (rev)

(rev '(a b c d e))
@result{} [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)
@end lisp

Note the way Guile indents the output, illustrating the depth of
execution at each procedure call.  This can be used to demonstrate, for
example, that Guile implements self-tail-recursion properly:
 
@lisp
(define (rev ls sl)
  (if (null? ls)
      sl
      (rev (cdr ls)
           (cons (car ls) sl)))) @result{} rev
 
(trace rev) @result{} (rev)
 
(rev '(a b c d e) '())
@result{} [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)
@end lisp
 
Since the tail call is effectively optimized to a @code{goto} statement,
there is no need for Guile to create a new stack frame for each
iteration.  Tracing reveals this optimization in operation.


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