#' Loop over draws
#'
#' Executes an expression once for every draw in a `draws` object. Used
#' primarily for its side effects and returns the input `x` invisibly.
#'
#' @template args-methods-x
#' @param expr (expression) A bare expression that can contain references to
#' variables in `x` by name. This expression will be executed once per draw
#' of `x`, where references to variables in `x` resolve to the value of that
#' variable in that draw. The expression supports
#' [quasiquotation][rlang::quasiquotation].
#'
#' @details
#' If `x` is not in the [`draws_rvars`] format, it is first converted to that
#' format. This allows the variables in `x` to include their dimensions (i.e,
#' to act as R vectors and arrays) when being referred to in `expr`.
#'
#' Within `expr`, use `.draw` to refer to the draw index, which will be a value
#' between 1 and `ndraws(x)`. `expr` is executed in the calling environment of
#' `for_each_draw()`, so it can use variables in that environment (however, due
#' to the use of data masking, to modify variables in that environment, one
#' must use `<<-`.)
#'
#' @returns
#' As `for_each_draw()` is used primarily for its side effects (the expression
#' executed for each draw of `x`), it returns the input `x` invisibly.
#'
#' @examples
#' eight_schools <- as_draws_rvars(example_draws())
#'
#'
#' # 1. A simple example --- looping over draws and printing each draw
#' # NOTE: You probably don't want to do this in practice! This example is
#' # just intended to show what for_each_draw() is doing. If you just want to
#' # print the draws of an rvar, it is probably better to use draws_of()
#' for_each_draw(eight_schools, {
#'   print(mu)
#' })
#'
#'
#' # 2. A more complex example --- building a parallel coordinates plot
#' # First, construct the plot bounds
#' plot(1, type = "n",
#'   xlim = c(1, length(eight_schools$theta)),
#'   ylim = range(range(eight_schools$theta)),
#'   xlab = "school", ylab = "theta"
#' )
#'
#' # Then, use for_each_draw() to make a parallel coordinates plot of all draws
#' # of eight_schools$theta. Use resample_draws(eight_schools, n = ...)
#' # in place of eight_schools if a smaller sample is desired for the plot.
#' for_each_draw(eight_schools, {
#'   lines(seq_along(theta), theta, col = rgb(1, 0, 0, 0.05))
#' })
#'
#' # Finally, add means and 90% intervals
#' lines(seq_along(eight_schools$theta), mean(eight_schools$theta))
#' with(summarise_draws(eight_schools$theta),
#'   segments(seq_along(eight_schools$theta), y0 = q5, y1 = q95)
#' )
#' @importFrom rlang eval_tidy
#' @export
for_each_draw <- function(x, expr) {
  x_rvars <- as_draws_rvars(x)
  expr <- enquo(expr)
  env <- parent.frame()

  for (i in seq_len(ndraws(x_rvars))) {
    variables <- get_variables_from_one_draw(x_rvars, i)
    variables$.draw = i
    eval_tidy(expr, data = variables, env = env)
  }

  invisible(x)
}
