validate mapping before data in layer(). closes #2862 (#2871)

Author: clauswilke

R/layer.r

@@ -81,12 +81,16 @@ layer <- function(geom = NULL, stat = NULL,
     show.legend <- FALSE
   }
 
-  data <- fortify(data)
-
+  # we validate mapping before data because in geoms and stats
+  # the mapping is listed before the data argument; this causes
+  # less confusing error messages when layers are accidentally
+  # piped into each other
   if (!is.null(mapping)) {
     mapping <- validate_mapping(mapping)
   }
 
+  data <- fortify(data)
+
   geom <- check_subclass(geom, "Geom", env = parent.frame())
   stat <- check_subclass(stat, "Stat", env = parent.frame())
   position <- check_subclass(position, "Position", env = parent.frame())