11. Debugging Features

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 if and 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 35. Debugging Infrastructure, 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?]

11.1 Single-Step		Execute a program or function one step at a time.
11.2 Trace		Print a report each time a given function is called.
11.3 Backtrace		See a list of the statements that caused an error.

11.1 Single-Step

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.

11.3 Backtrace

Scheme Procedure: backtrace

C Function: scm_backtrace ()

Display a backtrace of the stack saved by the last error to the current output port.