使用let in和模式匹配的SML merge_sort函数

fun merge_sort [] = []
  | merge_sort [x] = [x]
  | merge_sort xs =
    let
      fun merge (xs, []) = xs
        | merge ([], ys) = ys
        | merge (x::xs, y::ys) =
          if x <= y then x::(merge (xs, y::ys))
          else y::(merge (x::xs, ys))

      fun split xs =
        let
          fun split' (xs, [], []) = (xs, [])
            | split' (x::xs, [y], [z]) = (x::xs, [y])
            | split' (x::y::xs, ys, zs) =
              let
                val (xs', ys') = split' (xs, ys, zs)
              in
                (x::xs', y::ys')
              end
        in
          split' (xs, [], [])
        end

      val (left, right) = split xs
    in
      merge (merge_sort left, merge_sort right)
    end;