(* 目的：lst1 と lst2 の長さが等しいかどうかを判定する *) 
(* equal_length : 'a list -> 'a list -> bool *) 
let rec equal_length lst1 lst2 = match (lst1, lst2) with 
    ([], []) -> true 
  | ([], first2 :: rest2) -> false 
  | (first1 :: rest1, []) -> false 
  | (first1 :: rest1, first2 :: rest2) -> equal_length rest1 rest2 
 
(* テスト *) 
let test1 = equal_length [] [] = true 
let test2 = equal_length [] [1; 2] = false 
let test3 = equal_length [1; 2] [] = false 
let test4 = equal_length [1; 3] [2; 4] = true 
let test5 = equal_length ["a"; "b"; "c"; "d"] [1; 3] = false 
let test6 = equal_length ["a"; "b"; "c"; "d"] [1; 3; 2; 4] = true