Merge pull request #9 from dajmcdon/epi_slide-f-param-doc

epi_slide `f` parameter documentation

Author: dajmcdon

R/slide.R

@@ -84,7 +84,27 @@
 #'   tidy evaluation (first example, above), then the name for the new column is 
 #'   inferred from the given expression and overrides any name passed explicitly 
 #'   through the `new_col_name` argument.
-#' 
+#'   
+#' When `f` is a named function with arguments, if a tibble with an unnamed 
+#'   grouping variable is passed in as the method argument to `f`, include a 
+#'   parameter for the grouping-variable in `function()` just prior to 
+#'   specifying the method to prevent that from being overridden. For example:
+#'   ```
+#'   # Construct an tibble with an unnamed grouping variable
+#'   edf = bind_rows(tibble(geo_value = "ak", time_value = as.Date("2020-01-01") 
+#'             + 1:10, x1=1:10, y=1:10 + rnorm(10L))) %>% 
+#'     as_epi_df()
+#'   
+#'   # Now, include a row parameter for the grouping variable in the tibble, 
+#'   # which we denote as g, just prior to method = "qr"
+#'   # Note that if g was not included below, then the method = "qr" would be 
+#'   # overridden, as described above
+#'   edf %>%
+#'   group_by(geo_value) %>%
+#'   epi_slide(function(x, g, method="qr", ...) tibble(model=list(
+#'             lm(y ~ x1, x, method=method))), n=7L) 
+#'   ```
+#'   
 #' @importFrom lubridate days weeks
 #' @importFrom rlang .data .env !! enquo enquos sym
 #' @export