如果在深镜头查找/分配中某处遇到“Nothing”，是否可以设置默认值？

Question

如果在深镜头查找/分配中某处遇到Nothing，是否可以设置默认值？

E.g。查找：

(Just (4, 3), 2) ^. _1 . (maybe (6, 5)) . _1 == 4
(Nothing,     2) ^. _1 . (maybe (6, 5)) . _1 == 6

或者更重要的是，对于作业：

((Just (4, 3), 2) & _1 . (maybe (6, 5)) . _1 .~ 7) == (Just (7, 3), 2)
((Nothing,     2) & _1 . (maybe (6, 5)) . _1 .~ 7) == (Just (7, 5), 2)

Answer 1

您可以使用non :: Eq a => a -> Iso' (Maybe a) a在Iso和Eq a => Maybe a (see also)之间创建Eq => a。

(Nothing, 2) ^. _1 . non (6, 5) . _1 == 6
((Nothing, 2) & _1 . non (6, 5) . _1 .~ 7) == (Just (7, 5), 2)

Answer 2

是的，似乎有可能：

let
  df :: Functor f => a -> (a -> f b) -> Maybe a -> f (Maybe b)
  df d = lens (maybe d id) (const Just)

(Just (4, 3), 2) ^. _1 . df (6, 5) . _1 `shouldBe` 4
(Nothing,     2) ^. _1 . df (6, 5) . _1 `shouldBe` 6

((Nothing,     2) & _1 . df (6, 5) . _1 .~ 7) `shouldBe` (Just (7, 5), 2)
((Just (4, 3), 2) & _1 . df (6, 5) . _1 .~ 7) `shouldBe` (Just (7, 3), 2)

注意：我正在熟悉镜片，所以如果有人知道更好的方法，请告诉 - 例如如果有一个等同于df的内置函数。