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#' Select variable
#'
#' This function powers [dplyr::pull()] and various functions of the
#' tidyr package. It is similar to [vars_select()] but returns only
#' one column name and has slightly different semantics: it allows
#' negative numbers to select columns from the end.
#'
#' @inheritParams vars_select
#' @param var A variable specified as:
#' * a literal variable name
#' * a positive integer, giving the position counting from the left
#' * a negative integer, giving the position counting from the right.
#'
#' The default returns the last column (on the assumption that's the
#' column you've created most recently).
#'
#' This argument is taken by expression and supports
#' [quasiquotation][rlang::quasiquotation] (you can unquote column
#' names and column locations).
#' @return The selected column name as an unnamed string.
#' @seealso [dplyr::pull()], [vars_select()]
#' @export
#' @keywords internal
#' @examples
#' # It takes its argument by expression:
#' vars_pull(letters, c)
#'
#' # Negative numbers select from the end:
#' vars_pull(letters, -3)
#'
#' # You can unquote variables:
#' var <- 10
#' vars_pull(letters, !! var)
vars_pull <- function(vars, var = -1) {
n <- length(vars)
instrument_base_errors(
loc <- eval_tidy(enquo(var), set_names(seq_along(vars), vars))
)
loc <- pull_as_location2(loc, n, vars)
if (loc < 0L) {
loc <- n + 1L + loc
}
vars[[loc]]
}
pull_as_location2 <- function(i, n, names) {
with_subscript_errors(type = "pull", {
i <- vctrs::vec_as_subscript2(i, arg = "var", logical = "error")
if (is.numeric(i)) {
vctrs::num_as_location2(
i,
n = n,
negative = "ignore",
arg = "var"
)
} else {
vctrs::vec_as_location2(
i,
n = n,
names = names,
arg = "var"
)
}
})
}
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