rot90.sml

val m = [[1, 2, 3], [6, 5, 4]];

fun rot90 (matrix) =
  let
      fun rot90 (m, acc) =
        if null (hd m) then
            rev acc
        else
            rot90 (map tl m, map hd m :: acc)
  in
      rot90 (matrix, nil)
  end;

rot90 (m)