(* 八百屋においてある野菜と値段のリストの例 *) 
let yaoya_list = [("トマト", 300); ("たまねぎ", 200); 
                  ("にんじん", 150); ("ほうれん草", 200)] 
 
(* 目的：item の値段を調べる *) 
(* price : string -> (string * int) list -> int option *) 
let rec price item yaoya_list = match yaoya_list with 
    [] -> None 
  | (yasai, nedan) :: rest -> 
      if item = yasai then Some (nedan) 
                      else price item rest 
 
(* テスト *) 
let test1 = price "トマト" yaoya_list = Some (300) 
let test2 = price "じゃがいも" yaoya_list = None