let transpose (matrix: 'a list list) = 

    let rec loop x number count = 

        if number < count then

            loop (List.foldBack(fun x acc -> (List.nth x number)::acc) matrix []::x) (number + 1) count

        else x

    if matrix.IsEmpty then []

    else loop [] 0 matrix.Head.Length |> List.rev

​

​

[[4;3];[1;5];[6;7]] |> transpose |> printfn "%A"

[[4;3;2];[1;5;0];[6;7;9];[4;3;2];[1;5;0]] |> transpose |> printfn "%A"

[] |> transpose |> printfn "%A"