`sig
val to_int : string -> int option
val to_float : string -> float option
val index : string -> char -> int option
val rindex : string -> char -> int option
val index_from : string -> int -> char -> int option
val rindex_from : string -> int -> char -> int option
val find : string -> string -> int option
val find_from : string -> int -> string -> int option
val rfind : string -> string -> int option
val rfind_from : string -> int -> string -> int option
val split : string -> by:string -> (string * string) option
val rsplit : string -> by:string -> (string * string) option
end`