let n = 4



let myList1 = ["aaa"; "b"; "ccc"]



let rec func (list : string list) (myList : string list) =
    match list with
    | head :: tail -> match myList with
                      | myHead :: myTail -> if myHead.Length + head.Length <= n then
                                                func tail ((myHead + head) :: myTail)
                                            else func tail (head :: myList)
                      | [] -> func tail [head]
    | [] -> myList |> List.rev

let myList2 = func myList1 []

printfn "%A" myList2