Improve missing facetting variable error message

Fixes #2517

Author: hadley

R/facet-.r

@@ -530,11 +530,20 @@ combine_vars <- function(data, env = emptyenv(), vars = NULL, drop = TRUE) {
   # For each layer, compute the facet values
   values <- compact(plyr::llply(data, eval_facets, facets = vars, env = env))
 
-  # Form the base data frame which contains all combinations of faceting
+  # Form the base data.frame which contains all combinations of faceting
   # variables that appear in the data
-  has_all <- unlist(plyr::llply(values, length)) == length(vars)
+  has_all <- unlist(lapply(values, length)) == length(vars)
   if (!any(has_all)) {
-    stop("At least one layer must contain all variables used for faceting")
+    missing <- lapply(values, function(x) setdiff(names(vars), names(x)))
+    missing_txt <- vapply(missing, var_list, character(1))
+    name <- c("Plot", paste0("Layer ", seq_len(length(data) - 1)))
+
+    stop(
+      "At least one layer must contain all faceting variables: ",
+      var_list(names(vars)), ".\n",
+      paste0("* ", name, " is missing ", missing_txt, collapse = "\n"),
+      call. = FALSE
+    )
   }
 
   base <- unique(plyr::ldply(values[has_all]))

R/utilities.r

@@ -360,6 +360,15 @@ warning_wrap <- function(...) {
   warning(paste0(wrapped, collapse = "\n"), call. = FALSE)
 }
 
+var_list <- function(x) {
+  x <- encodeString(x, quote = "`")
+  if (length(x) > 5) {
+    x <- c(x[1:5], paste0("and ", length(x) - 5, " more"))
+  }
+
+  paste0(x, collapse = ", ")
+}
+
 dispatch_args <- function(f, ...) {
   args <- list(...)
   formals <- formals(f)