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  expression (such as returned by a call to \code{\link{quote}} or

  expression (such as returned by a call to \code{\link{quote}} or

  \code{\link{substitute}}).  This expression itself is inserted in the

  \code{\link{substitute}}).  This expression itself is inserted in the

  traced function, so it will typically involve arguments or local

  traced function, so it will typically involve arguments or local

  objects in the traced function.  An expression of this form is useful

  objects in the traced function.  An expression of this form is useful

  if you only want to interact when certain conditions apply (and in

  if you only want to interact when certain conditions apply (and in

  to \code{trace} also).

  to \code{trace} also).

  When the \code{at} argument is supplied, it can be a vector of

  When the \code{at} argument is supplied, it can be a vector of

  integers referring to the substeps of the body of the function (this

  integers referring to the substeps of the body of the function (this

  only works if the body of the function is enclosed in \code{{ ...}}.  In

  only works if the body of the function is enclosed in \code{{ ...}}.  In

  \code{untrace} on the same function or method in between, the

  \code{untrace} on the same function or method in between, the

  previously edited version is retained.  If you want to throw away

  previously edited version is retained.  If you want to throw away

  all the previous tracing and then edit, call \code{untrace} before the next

  all the previous tracing and then edit, call \code{untrace} before the next

  call to \code{trace}.  Editing may be combined with automatic

  call to \code{trace}.  Editing may be combined with automatic

  tracing; just supply the other arguments such as \code{tracer}, and

  tracing; just supply the other arguments such as \code{tracer}, and

  uses the default editor (see \code{\link{edit}}).

  uses the default editor (see \code{\link{edit}}).

  Tracing primitive functions (builtins and specials) from the base

  Tracing primitive functions (builtins and specials) from the base

  package works, but only by a special mechanism and not very

  package works, but only by a special mechanism and not very

  informatively.  Tracing a primitive causes the primitive to be

  informatively.  Tracing a primitive causes the primitive to be

    if (x > 0) x ^ y else stop("x must be positive")

    if (x > 0) x ^ y else stop("x must be positive")

}

}

## arrange to call the browser on entering and exiting

## arrange to call the browser on entering and exiting

## function f

## function f

## instead, conditionally assign some data, and then browse

## instead, conditionally assign some data, and then browse

## on exit, but only then.  Don't bother me otherwise

## on exit, but only then.  Don't bother me otherwise

trace("f", quote(if(any(y < 0)) yOrig <- y),

trace("f", quote(if(any(y < 0)) yOrig <- y),

      print = FALSE)

      print = FALSE)

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

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

## the step numbers

## the step numbers

as.list(body(f))

as.list(body(f))

as.list(body(f)[[3]])

as.list(body(f)[[3]])

## Now call the browser there

## Now call the browser there

## trace a utility function, with recover so we

## trace a utility function, with recover so we

## can browse in the calling functions as well.

## can browse in the calling functions as well.

trace("as.matrix", recover)

trace("as.matrix", recover)