diff --git a/R/datatype.R b/R/datatype.R
index 0dda37b59..4bac46913 100644
--- a/R/datatype.R
+++ b/R/datatype.R
@@ -168,7 +168,7 @@ DataType_constructors = list(
   },
 
   # doc below pl_List
-  List = function(datatype) {
+  List = function(datatype = "unknown") {
     if (is.character(datatype) && length(datatype) == 1) {
       datatype = .pr$DataType$new(datatype)
     }
@@ -185,7 +185,7 @@ DataType_constructors = list(
   Struct = function(...) {
     result({
       largs = list2(...)
-      if (is.list(largs[[1]])) {
+      if (length(largs) >= 1 && is.list(largs[[1]])) {
         largs = largs[[1]]
         element_name = "list element"
       } else {
@@ -239,13 +239,27 @@ NULL
 #'
 #' # Find any DataType via pl$dtypes
 #' print(pl$dtypes)
+#'
+#' # check if an element is any kind of Struct()
+#' test = pl$Struct(pl$UInt64)
+#' pl$same_outer_dt(test, pl$Struct())
+#'
+#' # `test` is a type of Struct, but it doesn't mean it is equal to an empty Struct
+#' test == pl$Struct()
 NULL
 
 #' Create List DataType
 #' @keywords pl
 #' @name pl_List
-#' @param datatype an inner DataType
+#' @param datatype an inner DataType, default is "Unknown" (placeholder for when inner DataType
+#' does not matter, e.g. as used in example)
 #' @return a list DataType with an inner DataType
 #' @format function
-#' @examples pl$List(pl$List(pl$Boolean))
+#' @examples
+#' # some nested List
+#' pl$List(pl$List(pl$Boolean))
+#'
+#' # check if some maybe_list is a List DataType
+#' maybe_List = pl$List(pl$UInt64)
+#' pl$same_outer_dt(maybe_List, pl$List())
 NULL
diff --git a/man/pl_List.Rd b/man/pl_List.Rd
index bbb3f9693..f22125f95 100644
--- a/man/pl_List.Rd
+++ b/man/pl_List.Rd
@@ -7,7 +7,8 @@
 function
 }
 \arguments{
-\item{datatype}{an inner DataType}
+\item{datatype}{an inner DataType, default is "Unknown" (placeholder for when inner DataType
+does not matter, e.g. as used in example)}
 }
 \value{
 a list DataType with an inner DataType
@@ -16,6 +17,11 @@ a list DataType with an inner DataType
 Create List DataType
 }
 \examples{
+# some nested List
 pl$List(pl$List(pl$Boolean))
+
+# check if some maybe_list is a List DataType
+maybe_List = pl$List(pl$UInt64)
+pl$same_outer_dt(maybe_List, pl$List())
 }
 \keyword{pl}
diff --git a/man/pl_Struct_datatype.Rd b/man/pl_Struct_datatype.Rd
index 27a6ac48d..bf17dc9ea 100644
--- a/man/pl_Struct_datatype.Rd
+++ b/man/pl_Struct_datatype.Rd
@@ -21,4 +21,11 @@ pl$List(pl$List(pl$Boolean))
 
 # Find any DataType via pl$dtypes
 print(pl$dtypes)
+
+# check if an element is any kind of Struct()
+test = pl$Struct(pl$UInt64)
+pl$same_outer_dt(test, pl$Struct())
+
+# `test` is a type of Struct, but it doesn't mean it is equal to an empty Struct
+test == pl$Struct()
 }