type LineElement =
    | Word of string
    | Space of int

let n = 5

let myList1 = ["aa"; "bb"; "cc"]

let rec func (list : string list) (myList : LineElement list list) =
    match list with
    | head :: tail -> match myList with
                      | [Word x; Space y] :: myTail -> if x.Length + head.Length <= n then
                                                           let newString = x + " " + head
                                                           func tail ([Word newString; Space (n - newString.Length)] :: myTail)
                                                       else func tail ([Word head; Space (n - head.Length)] :: myList)
                      | [] -> func tail [[Word head; Space (n - head.Length)]]
                      | _ -> failwith "Error"
    | [] -> myList |> List.rev

let myList2 = func myList1 []

printfn "%A" myList2