偶数の要素だけ抽出する even

c9_5.ml

(* 目的: 偶数の要素だけ抽出する *)
(* even : int list -> int -> list *)
let rec even xs =
  match xs with
    [] -> []
  | first :: rest ->
      if first mod 2 = 0
        then first :: even rest
        else even rest;;

(* テスト *)
let even1 = even []                  = [];;
let even2 = even [1]                 = [];;
let even3 = even [2]                 = [2];;
let even4 = even [1;2]               = [2];;
let even5 = even [1;2;3;4]           = [2;4];;
let even6 = even [1;2;3;4;5;6;7;8;9] = [2;4;6;8];;