Array flattener

Program.fs

type MyElement =
    | MyNumber of int
    | MyArray of MyElement list

let rec flatten = function
    | [] -> []
    | (MyNumber head) :: tail -> head :: flatten tail
    | (MyArray head) :: tail -> List.append (flatten head) (flatten tail)

[<EntryPoint>]
let main argv =
    [ MyArray [ MyNumber 1; MyNumber 2; MyArray [ MyNumber 3 ] ]; MyNumber 4 ]
    |> flatten
    |> printfn "%A"
    0