受け取ったリストの長さを返す関数length

c9_4.ml

(* 目的: 受け取ったリストの長さを返す関数length *)
(* length : 'a list -> int *)
let rec length xs =
  match xs with
    [] -> 0
  | first :: rest -> 1 + length rest;;

(* テスト *)
let length1 = length []            = 0;;
let length2 = length [1]           = 1;;
let length3 = length [1;2]         = 2;;
let length4 = length [1;2;3]       = 3;;
let length5 = length ["a"]         = 1;;
let length6 = length ["a";"b"]     = 2;;
let length7 = length ["a";"b";"c"] = 3;;