module type S = sig
  type ('a, 'b) t
  (** Implementation selected according to current OCaml version. *)

  (** This module defines the common [Either] interface that is provided for all
      OCaml versions. For documentation of these functions, refer to the
      standard library. *)

  val left : 'a -> ('a, 'b) t
  val right : 'b -> ('a, 'b) t
  val is_left : ('a, 'b) t -> bool
  val is_right : ('a, 'b) t -> bool
  val find_left : ('a, 'b) t -> 'a option
  val find_right : ('a, 'b) t -> 'b option
  val map_left : ('a1 -> 'a2) -> ('a1, 'b) t -> ('a2, 'b) t
  val map_right : ('b1 -> 'b2) -> ('a, 'b1) t -> ('a, 'b2) t

  val map :
    left:('a1 -> 'a2) -> right:('b1 -> 'b2) -> ('a1, 'b1) t -> ('a2, 'b2) t

  val fold : left:('a -> 'c) -> right:('b -> 'c) -> ('a, 'b) t -> 'c
  val iter : left:('a -> unit) -> right:('b -> unit) -> ('a, 'b) t -> unit
  val for_all : left:('a -> bool) -> right:('b -> bool) -> ('a, 'b) t -> bool

  val equal :
    left:('a -> 'a -> bool) ->
    right:('b -> 'b -> bool) ->
    ('a, 'b) t ->
    ('a, 'b) t ->
    bool

  val compare :
    left:('a -> 'a -> int) ->
    right:('b -> 'b -> int) ->
    ('a, 'b) t ->
    ('a, 'b) t ->
    int
end
