functor

functor.ml

type comparison = Less | Equal | Greater;;

module type ORDERED_TYPE =
  sig
    type t
    val compare: t -> t -> comparison
  end;;

module Set =
  functor (Elt: ORDERED_TYPE) ->
    struct
      type element = Elt.t
      type set = element list
      let empty = []
      let rec add x s =
        match s with
        | [] -> [x]
        | head::tail ->
            match Elt.compare x head with
            | Equal -> s (* x is already in s *)
            | Less -> x::s (* x is smaller than all elements of s *)
            | Greater -> head::add x tail
      let rec member x s =
        match s with
        | [] -> false
        | head::tail ->
            match Elt.compare x head with
            | Equal -> true (* x belongs to s *)
            | Less -> false (* x is smaller than all elements of s *)
            | Greater -> member x tail
    end;;

module OrderedString =
  struct
    type t = string
    let compare x y = if x = y then Equal else if x < y then Less else Greater
  end;;

module StringSet = Set(OrderedString);;

let ans = StringSet.member "bar" (StringSet.add "bar" StringSet.empty) in
if ans then print_string "true" else print_string "false"