仅当输入值为“ Just x”时才套用镜头“ setter”吗？

Question

当输入值（未设置字段）为？时，是否有任何方法可以应用Control-Lens-Setter.html＃g：4中提到的组合器/设置器？Just x

例如，考虑(~+)组合器，如果我具有以下条件：

let target = (1, 2)
    input1 = Just 10
    input2 = Nothing

我想要一个执行以下操作的设置器：

(11, 2) == target & (_1 . someSetter) +~ input1 
                  & (_2 . someSetter) +~ input2

我尝试避免使用fmap或maybe/fromMaybe在这种情况下使用，因为我有很多此类操作要做，并且我更喜欢通过利用镜片的简洁/简洁来避免样板。

Answer 1

您总是可以定义自己的设置器。

maybeSetter :: (b -> a -> a) -> ASetter s t a a -> Maybe b -> s -> t
maybeSetter g f x = runIdentity . f (Identity . maybe id g x)

(+!) :: Num a => ASetter s t a a -> Maybe a -> s -> t
(+!) = maybeSetter (+)

let target = (1, 2)
    input1 = Just 10
    input2 = Nothing
in
    (11, 2) == target & (_1 . someSetter) +! input1 
                      & (_2 . someSetter) +! input2

您还可以轻松定义其他设置器。

(-!) :: Num a => ASetter s t a a -> Maybe a -> s -> t
(-!) = maybeSetter subtract

(||!) :: ASetter s t Bool Bool -> Maybe Bool -> s -> t
(||!) = maybeSetter (||)

(<>!) :: Semigroup a => ASetter s t a a -> Maybe a -> s -> t
(<>!) = maybeSetter (flip (<>))

(.!) :: ASetter s t a a -> Maybe a -> s -> t
(.!) = maybeSetter const

(%!) :: ASetter s t a a -> Maybe (a -> a) -> s -> t
(%!) = maybeSetter id

那是关于镜头的很棒的事情。它们只是常规功能。