Clojureで継承みたいなナニカ

Clojureのdeftypeやdefrecordではinterfaceのみを実装することができます。
しかし、継承のように実装があるclassを継承したいときがあります。

例えばこんなコードがあります。

	(defprotocol Sequence
	(add [xs x])
	(empty [xs])
	(head [xs])
	(tail [xs]))
	(defrecord Cons [x xs]
	Sequence
	(add [xs x] (Cons. x xs))
	(empty [xs] (Nil.))
	(head [_] x)
	(tail [_] xs))
	(defrecord Nil []
	Sequence
	(add [xs x] (Cons. x xs))
	(empty [xs] (Nil.))
	(head [_] nil)
	(tail [xs] xs))
	(defrecord Wrapped [list]
	Sequence
	(add [xs x]
	(update-in xs [:list] (partial cons x)))
	(empty [xs]
	(Wrapped. '()))
	(head [xs]
	(first list))
	(tail [xs]
	(update-in xs [:list] rest)))

view raw sequence.clj hosted with ❤ by GitHub

ConsとNilのaddとemptyの実装が同じです。
共通化したい。

とりあえず、addとemptyを別Protocolに。

	(defprotocol Sequence
	(head [xs])
	(tail [xs]))
	(defprotocol Adder
	(add [xs x])
	(empty [xs]))

view raw adder.clj hosted with ❤ by GitHub

ListというProtocolを作って、それに対するAdderを定義します。

	(defprotocol List
	(list [xs]))
	(extend-type List
	Adder
	(add [xs x] (Cons. x xs))
	(empty [xs] (Nil.)))

view raw list.clj hosted with ❤ by GitHub

ConsとNilに対してListを実装すれば、めでたしめでたし。

	(defrecord Cons [x xs]
	List
	(list [xs] xs)
	Sequence
	(head [_] x)
	(tail [_] xs))
	(defrecord Nil []
	List
	(list [xs] xs)
	Sequence
	(head [_] nil)
	(tail [xs] xs))

view raw cons.clj hosted with ❤ by GitHub