OCaml How to reverse only even values in list

I want to transefer something like this:

[0,1,2,3,4,5,6,7]

into

[6,1,4,3,2,5,0,7]

Here's a draft, that reverses the elements at even positions (which is what I assume you meant by your question). This is probably far from optimal (e.g. you should make split and join tail-recursive):

let rec split = function
  | [] -> [], []
  | h::[] -> [h], []
  | x::y::t -> 
    let a, b = split t in 
    x::a, y::b;;

let rec join a b = match a, b with
  | [], _ -> b
  | _, [] -> a
  | ha::ta, hb::tb -> ha::hb::(join ta tb);;

let doit l = 
  let a, b = split l in 
  join a (List.rev b);;