Add language specialization example in R

examples/types/by-language/types.R

@@ -19,3 +19,33 @@ f <- function(x) x + 1
 f <- function(x){
   colSums(x)
 }
+
+
+## Two different language specific specializations of Table
+#-- RDataFrame ::
+#--   lang "R"
+#--   cast_as "data.frame"
+#--   is_a Table
+#-- RDataTable ::
+#--   lang "R"
+#--   cast_as "data.table"
+#--   import "data.table"
+#--   is_a Table
+
+## Define the casting function
+#-- as.data.table :: role "cast" => RDataFrame a -> RDataTable a
+#-- as.data.frame :: role "cast" => RDataTable a -> RDataFrame a
+
+## Finite set of symbols (Enum) but encoded in R as a string (rather than the
+## default, a factor). We avoid the issues of handling factors, but retain the
+## semantic information.
+#-- RStringFactor ::
+#--   lang "R"
+#--   cast_as "character"
+#--   is_a Enum
+
+#-- foo :: RDataFrame {a:Integer; b:String; c:RStringFactor (Cat, Dog, Eel)} ->
+#--        RDataFrame {a:Integer; b:String}
+foo <- function(x){
+  subset(x, c == "Eel")[-3]
+}