DouKaku?

let pyramid str =
  let rec letters = function
    | "" -> []
    | s  -> (String.sub s 0 1) :: (letters (String.sub s 1 (String.length s - 1)))
  in
  fst (List.fold_right (fun s t -> let x = s ^ snd t in (x :: (fst t), " " ^ x))
                       (letters str) ([], ""))
  |> List.mapi (fun i s -> (String.make i ' ') ^ s) |> List.rev |> String.concat "\n";;

let _ = pyramid "abracadabra" |> printf "%s\n";;