haskell - 在镜头库中使用Traversable []和Applicative Maybe的效果

Question

我有以下结构:

y = [
  fromList([("c", 1 ::Int)]),
  fromList([("c", 5)]),
  fromList([("d", 20)])
  ]

我可以用它来更新每个“c”:

y & mapped . at "c" . mapped  %~ (+ 1)
-- [fromList [("c",2)], fromList [("c",6)], fromList [("d",20)]]

所以第三个条目基本上被忽略了。但我想要的是操作失败。

Only update, iff all the maps contain the key "c".

所以我想要:

y & mysteryOp
-- [fromList [("c",1)], fromList [("c",5)], fromList [("d",20)]]
-- fail because third entry does not contain "c" as key

我想我知道在这里使用哪些功能:

over
-- I want to map the content of the list

mapped
-- map over the structure and transform to [(Maybe Int)]

traverse
-- I need to apply the operation, which will avoid 

at "c"
-- I need to index into the key "c"

我只是不知道如何组合它们

Answer 1

这里有几种替代方法，可以根据您的喜好进行观察；

使用懒惰来延迟决定是否进行更改，

f y = res
  where (All c, res) = y 
                     & each %%~ (at "c" %%~ (Wrapped . is _Just &&& fmap (applyWhen c succ)))

或者预先决定是否进行更改，

f' y = under (anon y $ anyOf each (nullOf $ ix "c")) (mapped . mapped . ix "c" +~ 1) y