Continuations.hs

callCC ::
    (MonadFreer c fr, Monad (fr f), c (FreerChurch f)) =>
    (forall r. (a -> fr f r) -> fr f a) ->
    fr f a
callCC f = reencodeFreer $ callCcChurch \exit -> reencodeFreer $ f (reencodeFreer . exit)

callCcChurch :: (forall r. (a -> FreerChurch f r) -> FreerChurch f a) -> FreerChurch f a
callCcChurch f = FreerChurch $ FT \k g -> runFT (unFreerChurch $ f $ pure . runIdentity . k) k g

getCC :: (MonadFreer c fr, Monad (fr f), c (FreerChurch f)) => fr f (fr f ())
getCC = callCC \exit -> let a = void $ exit a in pure a

reencodeFreer :: (Freer c fr, Freer c' fr', c (fr' f)) => fr f ~> fr' f
reencodeFreer = interpretFreer liftIns

FreerChurch: ChurchエンコードされたFreerの型
reencodeFreer: Freerのエンコーディングを別のものへ変換する
Freer c, MonadFreer: エンコーディングを捨象した一般のFreerを表現する型クラス

FTはfreeパッケージのもの