www.delorie.com/gnu/docs/guile/guile_38.html

search

	Buy GNU books!

Guile Reference Manual

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.

Function: trace 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.

Function: untrace 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.