+module type CELL = sig
+ type t
+
+ val state : t -> int
+
+ val react : t -> states:int list -> t
+end
+
+
+module Conway : CELL = struct
+ type t = D | A
+
+ let state = function
+ | D -> 0
+ | A -> 1
+
+ let react t ~states =
+ let live_neighbors = List.fold_left states ~init:0 ~f:(+) in
+ match t with
+ | A when live_neighbors < 2 -> D
+ | A when live_neighbors < 4 -> A
+ | A when live_neighbors > 3 -> D
+ | D when live_neighbors = 3 -> A
+ | t -> t
+end
+
+