Traspose now!

transpose.ml

let rec transpose xss = 
  match xss with 
  | [] :: _ | [] -> []
  | xs :: xss -> 
    let hds = List.map List.hd xss in 
    let tls = List.map List.tl xss in
    let hd = List.hd xs in 
    let tl = List.tl xs in 
    (hd :: hds) :: transpose (tl :: tls) 

let rec transpose1 xss = 
  match xss with 
  | [] -> [] (* we would only get here if we're actually given an empty list *)
  | [xs] -> xs |> List.map (fun elem -> [elem])
  | xs :: xss -> 
    let almost = transpose1 xss in 
    List.combine xs almost 
    |> List.map (fun (x, xs) -> x :: xs)