Mixing closures with abstract datatypes

	(* abstract data types with closures *)
	datatype set = S of { insert : int -> set,
	member : int -> bool,
	size : unit -> int }
	val empty_set =
	let
	fun make_set xs = (* xs is a private field in result *)
	let
	fun contains i = List.exists (fn j => i=j) xs
	in
	S { insert = fn i => if contains i
	then make_set xs
	else make_set (i::xs),
	member = contains,
	size = fn () => length xs
	}
	end
	in
	make_set []
	end
	(* example of using our newly described set *)
	fun use_sets() =
	let val S s1 = empty_set
	val S s2 = (#insert s1) 34
	val S s3 = (#insert s2) 34
	val S s4 = #insert s3 19
	in
	if (#member s4) 42
	then 99
	else if (#member s4) 19
	then 17 + (#size s3) ()
	else 0
	end

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