nonempty.ml

module Nonempty : sig
  type 'a t
  val init : 'a -> 'a t
  val create : 'a -> 'a list -> 'a t
  val head : 'a t -> 'a
  val extend : 'a t -> 'a list -> 'a t
  val replace: 'a t -> 'a list -> 'a t
  val merge: 'a t -> 'a t -> 'a t
  val of_list : 'a list -> 'a t option
  val to_list : 'a t -> 'a list
  val map : ('a -> 'b) -> 'a t -> 'b t
end = struct
  type 'a t = Nonempty of 'a * 'a list

  let create t more =
    Nonempty (t, more)

  let init a =
    create a []

  let head (Nonempty (t, more)) = t

  let extend (Nonempty (t, more)) extras =
    create t (more @ extras)

  let replace t replacement =
    create (head t) replacement

  let of_list = function
    | [] -> None
    | t :: more -> Some (create t more)

  let to_list (Nonempty (t, more)) =
    t :: more

  let merge a b =
    extend a @@ to_list b

  let map f (Nonempty (t, more)) =
    create (f t) (List.map f more)
end