17 Debugging
17.1 Tracing
The racket/trace library mimics the tracing facility
available in Chez Scheme.
Each
id must be bound to a procedure in the environment of
the
trace expression. Each
id is
set!ed to
a new procedure that traces procedure calls and returns by printing
the arguments and results of the call via
current-trace-notify. If multiple values are returned, each
value is displayed starting on a separate line.
When traced procedures invoke each other, nested invocations are shown
by printing a nesting prefix. If the nesting depth grows to ten and
beyond, a number is printed to show the actual nesting depth.
The trace form can be used on an identifier that is already
traced. In this case, assuming that the variable’s value has not been
changed, trace has no effect. If the variable has been
changed to a different procedure, then a new trace is installed.
Tracing respects tail calls to preserve loops, but its effect may be
visible through continuation marks. When a call to a traced procedure
occurs in tail position with respect to a previous traced call, then
the tailness of the call is preserved (and the result of the call is
not printed for the tail call, because the same result will be printed
for an enclosing call). Otherwise, however, the body of a traced
procedure is not evaluated in tail position with respect to a call to
the procedure.
The result of a trace expression is #<void>.
Examples: |
| > (define (f x) (if (zero? x) 0 (add1 (f (sub1 x))))) |
| > (trace f) |
| > (f 10) |
>(f 10) | > (f 9) | > >(f 8) | > > (f 7) | > > >(f 6) | > > > (f 5) | > > > >(f 4) | > > > > (f 3) | > > > > >(f 2) | > > > > > (f 1) | > > > >[10] (f 0) | < < < <[10] 0 | < < < < < 1 | < < < < <2 | < < < < 3 | < < < <4 | < < < 5 | < < <6 | < < 7 | < <8 | < 9 | <10 |
|
10 |
Undoes the effects of the
trace form for each
id,
set!ing each
id back to the untraced procedure, but
only if the current value of
id is a traced procedure. If
the current value of a
id is not a procedure installed by
trace, then the variable is not changed.
The result of an untrace expression is #<void>.
A parameter that determines the way that trace output is
displayed. The string given to
proc is a trace; it does not
end with a newline, but it may contain internal newlines. Each call or
result is converted into a string using
pretty-print. The
parameter’s default value prints the given string followed by a newline to
(current-output-port).
Calls
proc with the arguments supplied in
args, and possibly using keyword arguments. Also prints out the
trace information during the call, as described above in the docs for
trace, using
id as the name of
proc.
The value of this parameter is invoked to print out the arguments of a
traced call. It receives the name of the function, the function’s
ordinary arguments, its keywords, the values of the keywords, and a
number indicating the depth of the call.