等价于 Functor 或 Foldable 对于更高种类的类型

Question

考虑这段代码，使用 Functor 和 Foldable 类型类：

{-# 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

是否有一些类似的类型类集可以对更高种类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

我怀疑您正在寻找 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 mmorph，尽管我怀疑 rank2classes 可能对你更有用。

等价于 Functor 或 Foldable 对于更高种类的类型

Equivalent to Functor or Foldable for higher-kinded types

haskell

higher-kinded-types