| trace {base} | R Documentation |
A call to trace allows you to insert debugging code (e.g., a
call to browser or recover) at chosen
places in any function. A call to untrace cancels the tracing.
Specified methods can be traced the same way, without tracing all
calls to the function. Trace code can be any R expression.
trace(what, tracer, exit, at, print = TRUE, signature = NULL)
untrace(what)
what |
The name (quoted or not) of a function to be traced or untraced. More than one name can be given in the quoted form, and the same action will be applied to each one. |
tracer |
Either a function or an unevaluated expression. The
function will be called or the expression will be evaluated either
at the beginning of the call, or before those steps in the call
specified by the argument |
exit |
Either a function or an unevaluated expression. The function will be called or the expression will be evaluated on exiting the function. See the details section. |
at |
An optional numeric vector. If supplied, |
print |
If |
signature |
If this argument is supplied, it should be a
signature for a method for function |
The trace function operates by constructing a revised version
of the function (or of the method, if signature is supplied),
and assigning the new object back where the original was found.
If only the what argument is given, a line of trace printing is
produced for each call to the function (back compatible with the
earlier version of trace).
The object constructed by trace is from a class that extends
"function" and which contains the original, untraced version.
A call to untrace re-assigns this version.
If the argument tracer or exit is the name of a
function, the tracing expression will be a call to that function, with
no arguments. This is the easiest and most common case, with the
functions browser and recover the
likeliest candidates; the former browses in the frame of the function
being traced, and the latter allows browsing in any of the currently
active calls.
The tracer or exit argument can also be an unevaluated
expression (such as returned by a call to quote or
substitute). This expression itself is inserted in the
traced function, so it will typically involve arguments or local
objects in the traced function. An expression of this form is useful
if you only want to interact when certain conditions apply (and in
this case you probably want to supply print=FALSE in the call
to trace also).
When the at argument is supplied, it should be a vector of
integers referring to the substeps of the body of the function (this
only works if the body of the function is enclosed in { ...}. In
this case tracer is not called on entry, but instead
just before evauating each of the steps listed in at. (Hint:
you don't want to try to count the steps in the printed version of a
function; instead, look at as.list(body(f)) to get the numbers
associated with the steps in function f.)
An intrinsic limitation in the exit argument is that it won't
work if the function itself uses on.exit, since the existing
calls will override the one supplied by trace.
Tracing does not nest. Any call to trace replaces previously
traced versions of that function or method, and untrace always
restores an untraced version. (Allowing nested tracing has too many
potentials for confusion and for accidentally leaving traced versions
behind.)
Tracing primitive functions (builtins and specials) from the base
package works, but only by a special mechanism and not very
informatively. Tracing a primitive causes the primitive to be
replaced by a function with argument ... (only). You can get a bit
of information out, but not much. A warning message is issued when
trace is used on a primitive.
The practice of saving the traced version of the function back where
the function came from means that tracing carries over from one
session to another, if the traced function is saved in the
session image. (In the next session, untrace will remove the
tracing.) On the other hand, functions that were in a package, not in
the global environment, are not saved in the image, so tracing expires
with the session for such functions.
Tracing a method is basically just like tracing a function, with the
exception that the traced version is stored by a call to
setMethod rather than by direct assignment, and so is
the untraced version after a call to untrace.
The version of trace described here is largely compatible with
the version in S-Plus, although the two work by entirely different
mechanisms. The S-Plus trace uses the session frame, with the
result that tracing never carries over from one session to another (R
does not have a session frame). Another relevant distinction has
nothing directly to do with trace: The browser in S-Plus
allows changes to be made to the frame being browsed, and the changes
will persist after exiting the browser. The R browser allows changes,
but they disappear when the browser exits. This may be relevant in
that the S-Plus version allows you to experiment with code changes
interactively, but the R version does not. (A future revision may
include a “destructive” browser for R.)
The traced function(s) name(s). The relevant consequence is the assignment that takes place.
The version of function tracing that includes any of the arguments except for the function name requires the methods package (because it uses special classes of objects to store and restore versions of the traced functions).
In rare cases, this may change some behavior. In particular, although the methods package generally supports older styles of classes and methods, it is stricter in treating class attributes. Just occasionally, older code may stop on an error with the methods package attached, but not otherwise.
browser and recover, the likeliest
tracing functions;
also, quote and substitute for
constructing general expressions.
f <- function(x, y) {
y <- pmax(y, .001)
x ^ y
}
## arrange to call the browser on entering and exiting
## function f
trace("f", browser, exit = browser)
## instead, conditionally assign some data, and then browse
## on exit, but only then. Don't bother me otherwise
trace("f", quote(if(any(y < 0)) yOrig <- y),
exit = quote(if(exists("yOrig")) browser()),
print = FALSE)
## trace a utility function, with recover so we
## can browse in the calling functions as well.
trace("as.matrix", recover)
## turn off the tracing
untrace(c("f", "as.matrix"))