% File src/library/utils/man/debugger.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2011 R Core Team
% Distributed under GPL 2 or later

\name{debugger}
\alias{debugger}
\alias{dump.frames}
\title{Post-Mortem Debugging}
\description{
  Functions to dump the evaluation environments (frames) and to examine
  dumped frames.
}
\usage{
dump.frames(dumpto = "last.dump", to.file = FALSE)
debugger(dump = last.dump)
}
\arguments{
  \item{dumpto}{a character string. The name of the object or file to
    dump to.}
  \item{to.file}{logical. Should the dump be to an \R object or to a
    file?}
  \item{dump}{An \R dump object created by \code{dump.frames}.}
}
\details{
  To use post-mortem debugging, set the option \code{error} to be a call
  to \code{dump.frames}.  By default this dumps to an \R object
  \code{last.dump} in the workspace, but it can be set to dump to a
  file (a dump of the object produced by a call to \code{\link{save}}).
  The dumped object contain the call stack, the active environments and
  the last error message as returned by \code{\link{geterrmessage}}.

  When dumping to file, \code{dumpto} gives the name of the dumped
  object and the file name has \file{.rda} appended.

  A dump object of class \code{"dump.frames"} can be examined by calling
  \code{debugger}.  This will give the error message and a list of
  environments from which to select repeatedly.  When an environment is
  selected, it is copied and the \code{\link{browser}} called from
  within the copy.  Note that not all the information in the original
  frame will be available, e.g. promises which have not yet been
  evaluated and the contents of any \code{\dots} argument.

  If \code{dump.frames} is installed as the error handler, execution
  will continue even in non-interactive sessions.  See the examples for
  how to dump and then quit.
}
\value{
  Invisible \code{NULL}.
}
\note{
  Functions such as \code{\link{sys.parent}} and
  \code{\link{environment}} applied to closures will not work correctly
  inside \code{debugger}.

  If the error occurred when computing the default value of a formal
  argument the debugger will report \dQuote{recursive default argument
  reference} when trying to examine that environment.

  Of course post-mortem debugging will not work if \R is too damaged to
  produce and save the dump, for example if it has run out of workspace.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.
}
\seealso{
  \code{\link{options}} for setting \code{error} options;
  \code{\link{recover}} is an interactive debugger working similarly to
  \code{debugger} but directly after the error occurs.
}
\examples{
\dontrun{
options(error = quote(dump.frames("testdump", TRUE)))

f <- function() {
    g <- function() stop("test dump.frames")
    g()
}
f()   # will generate a dump on file "testdump.rda"
options(error = NULL)

## possibly in another R session
load("testdump.rda")
debugger(testdump)
Available environments had calls:
1: f()
2: g()
3: stop("test dump.frames")

Enter an environment number, or 0 to exit
Selection: 1
Browsing in the environment with call:
f()
Called from: debugger.look(ind)
Browse[1]> ls()
[1] "g"
Browse[1]> g
function() stop("test dump.frames")
<environment: 759818>
Browse[1]>
Available environments had calls:
1: f()
2: g()
3: stop("test dump.frames")

Enter an environment number, or 0 to exit
Selection: 0

## A possible setting for non-interactive sessions
options(error = quote({dump.frames(to.file = TRUE); q(status = 1)}))
}}
\keyword{utilities}
\keyword{error}