(* Language codes for celebration names. Two-letter lowercase ISO-639-1; the set
   is open so an overlay can add a language without a code change (spec §2.1). *)
type t = string

let of_string s =
  if String.length s <> 2 then
    Error (Printf.sprintf "lang %S: expected a 2-letter ISO-639-1 code" s)
  else if not (String.for_all (fun c -> c >= 'a' && c <= 'z') s) then
    Error (Printf.sprintf "lang %S: must be lowercase a-z" s)
  else Ok s

let of_string_exn s = match of_string s with Ok t -> t | Error e -> invalid_arg e
let to_string t = t
let compare = String.compare
let equal = String.equal

let sexp_of_t t = Sexplib0.Sexp_conv.sexp_of_string t

let t_of_sexp sexp =
  let s = Sexplib0.Sexp_conv.string_of_sexp sexp in
  match of_string s with
  | Ok t -> t
  | Error msg -> Sexplib0.Sexp_conv.of_sexp_error msg sexp