Functor implementing polymorphic module type... Help!

Learning
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3

You can’t do exactly what you want (define a polymorphic set) but something close:

(*first your signature for set should be*)
module type Set = sig
  type t (*the type of the sets*)
  type elt (*the type of the elements*)

  val empty : t

  val mem : elt -> t -> bool

  val add : elt -> t -> t

  val elts : t -> elt list
end

and then use your implementation with this type annotation:

module ListSet (M : Equal) : Set with type elt = M.t = struct

 (* put your implementation as above*)

end

Maybe this post could help you to understand a little bit more.