trace                  package:base                  R Documentation

Interactive Tracing and Debugging of Calls to a Function or Method

Description:

     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.  Tracing can be temporarily
     turned on or off globally by calling 'tracingState'.

Usage:

     trace(what, tracer, exit, at, print, signature, where = topenv(parent.frame()))
     untrace(what, signature = NULL, where = topenv(parent.frame()))

     tracingState(on = NULL)

Arguments:

    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 'at'. See the details section.

    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: optional numeric vector.  If supplied, 'tracer' will be
          called just before the corresponding step in the body of the
          function. See the details section. 

   print: If 'TRUE' (as per default), a descriptive line is printed
          before any trace expression is evaluated.

signature: If this argument is supplied, it should be a signature for a
          method for function 'what'.  In this case, the method, and
          _not_ the function itself, is traced.

   where: the environment from which to look for the function to be
          traced; by default, the top-level environment of the call to
          'trace'.  If you put a call to 'trace' into code in a
          package, you may need to specify 'where=.GlobalEnv' if the
          package containing the call has a namespace, but the function
          you want to trace is somewhere on the search list. 

      on: logical; a call to 'tracingState' returns 'TRUE' if tracing
          is globally turned on, 'FALSE' otherwise.  An argument of one
          or the other of those values sets the state.  If the tracing
          state is 'FALSE', none of the trace actions will actually
          occur (used, for example, by debugging functions to shut off
          tracing during debugging).

Details:

     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 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'.)

     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.)

Value:

     The traced function(s) name(s).  The relevant consequence is the
     assignment that takes place.

Note:

     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).

     If methods dispatch is not currently on, 'trace' will load the
     methods namespace, but will not put the methods package on the
     search list.

References:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

See Also:

     'browser' and 'recover', the likeliest tracing functions; also,
     'quote' and 'substitute' for constructing general expressions.

Examples:

     if(.isMethodsDispatchOn()) { # trace needs method package attached.

     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"))
     }

