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 can 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 evaluating 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.)
The at argument can also be a list of integer vectors. In
this case, each vector refers to a step nested within another step of
the function. For example, at = list(c(3,4))
will call the tracer just before the fourth step of the third step
of the function. See the example below.
Using setBreakpoint (from package utils) may be an
alternative, calling trace(...., at, ...).
The exit argument is called during on.exit
processing. In an on.exit expression, the experimental returnValue()
function may be called to obtain the value about to be returned by
the function. Calling this function in other circumstances will give
undefined results.
An intrinsic limitation in the exit argument is that it won't
work if the function itself uses on.exit with add=
FALSE (the default), 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 (except for edited
versions as discussed below), and untrace always
restores an untraced version. (Allowing nested tracing has too many
potentials for confusion and for accidentally leaving traced versions
behind.)
When the edit argument is used repeatedly with no call to
untrace on the same function or method in between, the
previously edited version is retained. If you want to throw away
all the previous tracing and then edit, call untrace before the next
call to trace. Editing may be combined with automatic
tracing; just supply the other arguments such as tracer, and
the edit argument as well. The edit = TRUE argument
uses the default editor (see edit).
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.)