#' Create a Formula from a Prepared Recipe
#'
#' In case a model formula is required, the formula method can
#'  be used on a recipe to show what predictors and outcome(s)
#'  could be used.
#'
#' @param x A recipe object that has been prepared.
#' @param ... Note currently used.
#' @return A formula.
#' @examples
#'
#' formula(recipe(Species + Sepal.Length ~ ., data = iris) %>% prep())
#'
#' iris_rec <- recipe(Species ~ ., data = iris) %>%
#'   step_center(all_numeric()) %>%
#'   prep()
#' formula(iris_rec)
#' @export
formula.recipe <- function(x, ...) {
  if (!fully_trained(x))
    rlang::abort(
      paste0(
      "The recipe must be prepped before the ",
      "formula can be computed."
      )
    )

  x <- summary(x)
  x_vars <- x$variable[x$role == "predictor"]
  x_vars <- x_vars[!is.na(x_vars)]
  if (length(x_vars) == 0)
    x_vars <- 1
  y_vars <- x$variable[x$role == "outcome"]
  y_vars <- y_vars[!is.na(y_vars)]
  if (length(y_vars) == 0)
    y_vars <- ""
  x_vars <- paste0(x_vars, collapse = "+")
  y_vars <- paste0(y_vars, collapse = "+")

  as.formula(paste(y_vars, x_vars, sep = "~"))
}
