feat : added condition to check the RHS of :=

R/data.table.R

@@ -1403,7 +1403,14 @@
     }
 
     if (!is.null(lhs)) {
-      # TODO?: use set() here now that it can add new columns. Then remove newnames and alloc logic above.
+      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 the function in `list()`).")
+        }
+      }
+      # 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))
     }

inst/tests/tests.Rraw

@@ -21157,3 +21157,8 @@ test(2317.6, DT1[DF1, on='a', .(d = x.a + i.d)]$d, 5)
 test(2317.7, DT1[DF2, on='a', e := i.e]$e, 5)
 test(2317.8, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)
 test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
+
+DT = data.table(x = 1:3)
+
+test(2318.1, DT[, y := mean], error = "RHS of `:=` is a function")
+test(2318.2, DT[, y := function(x) x], error = "RHS of `:=` is a function")