对于更高级的类型，等效于Functor或Foldable

Question

考虑使用Functor和Foldable类型类的代码：

{-# LANGUAGE DeriveFunctor, DeriveFoldable #-}
data Foo a = Foo (Maybe a) [a] deriving(Show, Functor, Foldable)
fmap (+1) (Foo (Just 1) [2,3,4]) -- result: Foo (Just 2) [3,4,5]
sum (Foo (Just 1) [2,3,4]) -- result: 10

有没有一组类似的类型类可以对更高级的类型参数进行操作？例如：

data Bar a = Bar (a Int) (a Bool)
somethingLikeFmap :: forall t f g. SomethingLikeFunctor t => (forall a. f a -> g a) -> t f -> t g
somethingLikeAll :: forall t f. SomethingLikeFoldable t => (forall a. f a -> Bool) -> t f -> Bool
somethingLikeFmap listToMaybe (Bar [1,2,3] [False, True]) -- desired result: Bar (Just 1) (Just False)
somethingLikeAll null (Bar [1,2,3] [False, True]) -- desired result: False
somethingLikeAll null (Bar [] []) -- desired result: True

Answer 1

我怀疑你在找rank2classes。例如：

{-# LANGUAGE FlexibleInstances, StandaloneDeriving, TemplateHaskell #-}
import Data.Maybe
import Data.Monoid
import qualified Rank2
import qualified Rank2.TH
data Bar a = Bar (a Int) (a Bool)
deriving instance Show (Bar Maybe)
Rank2.TH.deriveFunctor ''Bar
Rank2.TH.deriveFoldable ''Bar
main = do
print $ Rank2.fmap listToMaybe (Bar [1,2,3] [False, True])
print $ getAll (Rank2.foldMap (All . null) (Bar [1,2,3] [False, True]))
print $ getAll (Rank2.foldMap (All . null) (Bar [] []))

编辑：或者可能是mmorph，不过我怀疑rank2classes可能对您更有用。