include-semantics.ml

module type Set = sig
  type 'a t
  val empty : 'a t
  val isEmpty : 'a t -> bool
end

module ListSet : Set = struct
  type 'a t = 'a list
  let empty = []
  let isEmpty l = l == []
end

module ListSetExt = struct
  include ListSet
  (* `len` should not type check if ListSet.t is still abstract *)
  let len l = if isEmpty l then 0 else List.length l
  let elts = function
    | [] -> []
    | h::t -> h::t
end

(* this should not type check as well *)
let _ = ListSetExt.elts (ListSetExt.empty)