=LET(
   ARRAY, LAMBDA(dim, [init], LET(
     Y, LAMBDA(SELF, arr,
          LAMBDA([a], [b],
            IF(ISOMITTED(b),
               INDEX(arr, a),
               SELF(SELF, IF(SEQUENCE(ROWS(arr)) = a, b, arr))
            )
          )
        ),
     Y(Y, IF(SEQUENCE(dim), IF(ISOMITTED(init), "", init)))
   )),
   my_arr, ARRAY(5, 0),
   new_arr, my_arr(1, "val1")(4, "val4"),
   new_arr()
 )