trace

R Base 3.6.2

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 generic function. Trace code (tracer) can be any R expression. Tracing can be temporarily turned on or off globally by calling tracingState.

Syntax

trace(what, tracer, exit, at, print, signature,
      where = topenv(parent.frame()), edit = FALSE)
untrace(what, signature = NULL, where = topenv(parent.frame()))<p></p><p>tracingState(on = NULL)
.doTrace(expr, msg)
returnValue(default = NULL)</p>

Arguments

Parameter	Description
`what`	the name, possibly quote()d, of a function to be traced or untraced. For untrace or for trace with more than one argument, more than one name can be given in the quoted form, and the same action will be applied to each one. For “hidden” functions such as S3 methods in a namespace, where = * typically needs to be specified as well.
`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 or list. 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.
`edit`	For complicated tracing, such as tracing within a loop inside the function, you will need to insert the desired calls by editing the body of the function. If so, supply the edit argument either as TRUE, or as the name of the editor you want to use. Then trace() will call edit and use the version of the function after you edit it. See the details section for additional information.
`where`	where to look for the function to be traced; by default, the top-level environment of the call to trace. An important use of this argument is to trace functions from a package which are “hidden” or called from another package. The namespace mechanism imports the functions to be called (with the exception of functions in the base package). The functions being called are not the same objects seen from the top-level (in general, the imported packages may not even be attached). Therefore, you must ensure that the correct versions are being traced. The way to do this is to set argument where to a function in the namespace (or that namespace). The tracing computations will then start looking in the environment of that function (which will be the namespace of the corresponding package). (Yes, it's subtle, but the semantics here are central to how namespaces work in R.)
`on`	logical; a call to the support function 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).
`expr, msg`	arguments to the support function .doTrace, calls to which are inserted into the modified function or method: expr is the tracing action (such as a call to browser()), and msg is a string identifying the place where the trace action occurs.
`default`	If returnValue finds no return value (e.g. a function exited because of an error, restart or as a result of evaluating a return from a caller function), it will return default instead.

Return Value

In the simple version (just the first argument), trace returns an invisible NULL. Otherwise, the traced function(s) name(s). The relevant consequence is the assignment that takes place. untrace returns the function name invisibly. tracingState returns the current global tracing state, and possibly changes it. When called during on.exit processing, returnValue returns the value about to be returned by the exiting function. Behaviour in other circumstances is undefined.

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 be

Examples

Example

# NOT RUN {
require(stats)

##  Very simple use
trace(sum)
hist(rnorm(100)) # shows about 3-4 calls to sum()
untrace(sum)

## Show how pt() is called from inside power.t.test():
if(FALSE)
  trace(pt) ## would show ~20 calls, but we want to see more:
trace(pt, tracer = quote(cat(sprintf("tracing pt(*, ncp = %.15g)\n", ncp))),
      print = FALSE) # <- not showing typical extra
power.t.test(20, 1, power=0.8, sd=NULL)  ##--> showing the ncp root finding:
untrace(pt)

# }
# NOT RUN {
<!-- %% methods is loaded when needed -->
# }
# NOT RUN {
<!-- %% if(.isMethodsDispatchOn()) { # non-simple use needs 'methods' package -->
# }
# NOT RUN {
f <- function(x, y) {
    y <- pmax(y, 0.001)
    if (x > 0) x ^ y else stop("x must be positive")
}

## arrange to call the browser on entering and exiting
## function f
trace("f", quote(browser(skipCalls = 4)),
      exit = quote(browser(skipCalls = 4)))

## 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(skipCalls = 4)),
      print = FALSE)

## Enter the browser just before stop() is called.  First, find
## the step numbers

untrace(f) # (as it has changed f's body !)
as.list(body(f))
as.list(body(f)[[3]]) # -> stop(..) is [[4]]

## Now call the browser there

trace("f", quote(browser(skipCalls = 4)), at = list(c(3,4)))
# }
# NOT RUN {
f(-1,2) # --> enters browser just before stop(..)
# }
# NOT RUN {
## trace a utility function, with recover so we
## can browse in the calling functions as well.

trace("as.matrix", recover)

## turn off the tracing (that happened above)

untrace(c("f", "as.matrix"))

# }
# NOT RUN {
## Useful to find how system2() is called in a higher-up function:
trace(base::system2, quote(print(ls.str())))
# }
# NOT RUN {
##-------- Tracing hidden functions : need 'where = *'
##
## 'where' can be a function whose environment is meant:
trace(quote(ar.yw.default), where = ar)
a <- ar(rnorm(100)) # "Tracing ..."
untrace(quote(ar.yw.default), where = ar)

## trace() more than one function simultaneously:
##         expression(E1, E2, ...)  here is equivalent to
##          c(quote(E1), quote(E2), quote(.*), ..)
trace(expression(ar.yw, ar.yw.default), where = ar)
a <- ar(rnorm(100)) # --> 2 x "Tracing ..."
# and turn it off:
untrace(expression(ar.yw, ar.yw.default), where = ar)


# }
# NOT RUN {
## trace calls to the function lm() that come from
## the nlme package.
## (The function nlme is in that package, and the package
## has a namespace, so the where= argument must be used
## to get the right version of lm)

trace(lm, exit = recover, where = asNamespace("nlme"))
# }
# NOT RUN {
<!-- %%}% only if 'methods' -->
# }