% File src/library/base/man/sys.parent.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2021 R Core Team
% Distributed under GPL 2 or later

\name{sys.parent}
\alias{sys.parent}
\alias{sys.call}
\alias{sys.calls}
\alias{sys.frame}
\alias{sys.frames}
\alias{sys.nframe}
\alias{sys.function}
\alias{sys.parents}
\alias{sys.on.exit}
\alias{sys.status}
\alias{parent.frame}
\title{Functions to Access the Function Call Stack}
\description{
  These functions provide access to \code{\link{environment}}s
  (\sQuote{frames} in S terminology) associated with functions further
  up the calling stack.
}
\usage{
sys.call(which = 0)
sys.frame(which = 0)
sys.nframe()
sys.function(which = 0)
sys.parent(n = 1)

sys.calls()
sys.frames()
sys.parents()
sys.on.exit()
sys.status()
parent.frame(n = 1)
}
\arguments{
  \item{which}{the frame number if non-negative, the number of frames
    to go back if negative.}
  \item{n}{the number of generations to go back.  (See the
    \sQuote{Details} section.)}
}
\details{
  \code{\link{.GlobalEnv}} is given number 0 in the list of frames.
  Each subsequent function evaluation increases the frame stack by 1.
  The call, function definition and the environment for evaluation
  of that function are returned by \code{sys.call}, \code{sys.function}
  and \code{sys.frame} with the appropriate index.

  \code{sys.call}, \code{sys.function} and \code{sys.frame} accept
  integer values for the argument \code{which}.  Non-negative values of
  \code{which} are frame numbers starting from \code{\link{.GlobalEnv}}
  whereas negative values are counted back from the frame number of the
  current evaluation.

  The parent frame of a function evaluation is the environment in which
  the function was called.  It is not necessarily numbered one less than
  the frame number of the current evaluation, nor is it the environment
  within which the function was defined.  \code{sys.parent} returns the
  number of the parent frame if \code{n} is 1 (the default), the
  grandparent if \code{n} is 2, and so on.  See also the \sQuote{Note}.

  \code{sys.nframe} returns an integer, the number of the current frame
  as described in the first paragraph.

  \code{sys.calls} and \code{sys.frames} give a pairlist of all the
  active calls and frames, respectively, and \code{sys.parents} returns
  an integer vector of indices of the parent frames of each of those
  frames.

  Notice that even though the \code{sys.}\emph{xxx} functions (except
  \code{sys.status}) are interpreted, their contexts are not counted nor
  are they reported.  There is no access to them.

  \code{sys.status()} returns a list with components \code{sys.calls},
  \code{sys.parents} and \code{sys.frames}, the results of calls to
  those three functions (which will include the call to
  \code{sys.status}: see the first example).

  \code{sys.on.exit()} returns the expression stored for use by
  \code{\link{on.exit}} in the function currently being evaluated.
  (Note that this differs from S, which returns a list of expressions
  for the current frame and its parents.)

  \code{parent.frame(n)} is a convenient shorthand for
  \code{sys.frame(sys.parent(n))} (implemented slightly more efficiently).
}
\note{
  Strictly, \code{sys.parent} and \code{parent.frame} refer to the
  \emph{context} of the parent interpreted function.  So internal
  functions (which may or may not set contexts and so may or may not
  appear on the call stack) may not be counted, and S3 methods can also do
  surprising things.

  As an effect of lazy evaluation, these functions look at the call stack at
  the time they are evaluated, not at the time they are called.  Passing
  calls to them as function arguments is unlikely to be a good idea, but
  these functions still look at the call stack and count frames from the
  frame of the function evaluation from which they were called.

  Hence, when these functions are called to provide default values for
  function arguments, they are evaluated in the evaluation of the called
  function and they count frames accordingly (see e.g. the \code{envir}
  argument of \code{\link{eval}}).
}
\value{
  \code{sys.call} returns a call, \code{sys.function} a function
  definition, and \code{sys.frame} and \code{parent.frame} return an
  environment.

  For the other functions, see the \sQuote{Details} section.
}

\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.  (Not \code{parent.frame}.)
}
\seealso{
  \code{\link{eval}} for a usage of \code{sys.frame} and \code{parent.frame}.
}
\examples{\donttest{
require(utils)

## Note: the first two examples will give different results
## if run by example().
ff <- function(x) gg(x)
gg <- function(y) sys.status()
str(ff(1))

gg <- function(y) {
    ggg <- function() {
        cat("current frame is", sys.nframe(), "\n")
        cat("parents are", sys.parents(), "\n")
        print(sys.function(0)) # ggg
        print(sys.function(2)) # gg
    }
    if(y > 0) gg(y-1) else ggg()
}
gg(3)

t1 <- function() {
  aa <- "here"
  t2 <- function() {
    ## in frame 2 here
    cat("current frame is", sys.nframe(), "\n")
    str(sys.calls()) ## list with two components t1() and t2()
    cat("parents are frame numbers", sys.parents(), "\n") ## 0 1
    print(ls(envir = sys.frame(-1))) ## [1] "aa" "t2"
    invisible()
  }
  t2()
}
t1()

test.sys.on.exit <- function() {
  on.exit(print(1))
  ex <- sys.on.exit()
  str(ex)
  cat("exiting...\n")
}
test.sys.on.exit()
## gives 'language print(1)', prints 1 on exit

## An example where the parent is not the next frame up the stack
## since method dispatch uses a frame.
as.double.foo <- function(x)
{
    str(sys.calls())
    print(sys.frames())
    print(sys.parents())
    print(sys.frame(-1)); print(parent.frame())
    x
}
t2 <- function(x) as.double(x)
a <- structure(pi, class = "foo")
t2(a)
}}
\keyword{programming}
\keyword{data}