added condition to check the rhs

Author: venom1204

R/data.table.R

@@ -1403,6 +1403,16 @@ replace_dot_alias = function(e) {
     }
 
     if (!is.null(lhs)) {
+      # We only need to check if we are creating new columns.
+      newnames = setdiff(lhs, names(x))
+    
+      if (length(newnames) > 0) {
+        if (is.function(jval)) {      
+          stopf("RHS of `:=` is a function. To create a new column of functions, it must be a list column (e.g., wrap in `list()`).")
+        } else if (is.list(jval) && length(jval) == 1 && is.function(jval[[1L]])) {
+          stopf("RHS of `:=` is a length-1 list containing a function. `data.table` does not automatically recycle lists. To create a list-column, use `rep(list(myfun), .N)` or `list(replicate(.N, myfun))` to match the number of rows.")
+        }
+      }
       # TODO?: use set() here now that it can add new columns. Then remove newnames and alloc logic above.
       .Call(Cassign,x,irows,cols,newnames,jval)
       return(suppPrint(x))