class Functor m => Monad m where return :: a -> m a (>>=) :: (a -> m b) -> m a -> m b join :: m (m a) -> m a join = (>>= id) k >>= m = join $ fmap k m extract は return と、extend は >>= と、duplicate は join と、それぞれなんとなく反対になってるような気がしますよね?(m と w も上下反対の対応がありますねw) コモナドの感覚を掴むために具体的な実装を見てみましょう。 -- | List Zipper data Z a = Z [a] a [a] left, right :: Z a -> Z a left (Z (l:ls) c rs) = Z ls l (c:rs) right (Z ls c
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