When debugging a program, programmers often find it helpful to examine
the program's internal status while it runs: the values of internal
variables, the choices made in
cond statements, and
so forth. Guile Scheme provides a debugging interface that programmers
can use to single-step through Scheme functions and examine symbol
bindings. This is different from the section Internal Debugging Interface, which permits
programmers to debug the Guile interpreter itself. Most programmers
will be more interested in debugging their own Scheme programs than the
interpreter which evaluates them.
[FIXME: should we include examples of traditional debuggers and explain why they can't be used to debug interpreted Scheme or Lisp?]
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. 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.
(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
there is no need for Guile to create a new stack frame for each
trace here helps us see why this is so.
When a running program is interrupted, usually upon reaching an error or breakpoint, its state is represented by a stack of suspended function calls, each of which is called a frame. The programmer can learn more about the program's state at the point of interruption by inspecting and modifying these frames.
#tif obj is a calling stack.
Go to the first, previous, next, last section, table of contents.