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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.
Enable debug tracing on 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. If called with no args, return a list of the names of currently traced procedures.
Disable debug tracing for
procedure. If called with no args, disable tracing for all currently traced procedures.
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.