this is the future jane street wants. #NoToNil #protest #Option4Ever #foryou #foryoupage

Nilable.ml

type nil = Nil

(* This version is unsafe because it makes the assumption that index >= 0. *)
let rec get_unsafe : 'a. at:int -> from:'a list -> 'a | nil =
  fun ~at:index ~from:list ->
  match list with
  | [] -> Nil
  | first :: _ when index = 0 -> first
  | _ :: rest -> get_unsafe ~at:(index - 1) ~from:rest

let rec get : 'a. at:int -> from:'a list -> 'a | nil =
  fun ~at:index ~from:list ->
  if index >= 0
  then get_unsafe ~at:index ~from:list
  else get_unsafe ~at:(List.length list + index) ~from:list

let get_exn : 'a. at:int -> from:'a list -> 'a =
  fun ~at ~from ->
  let value = get ~at ~from in
  match type of value with
  | nil -> failwith "get_exn: index out of bounds"
  | _ -> value

To be clear, this is not valid OCaml code, it's just a (bad) proof of concept