对于更高级的类型，相当于 Functor 或 Foldable

问题描述

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) [2,4]) -- result: 10

是否有一些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,True]) -- desired result: False
somethingLikeAll null (Bar [] []) -- desired result: True

解决方法

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,True]))
    print $ getAll (Rank2.foldMap (All . null) (Bar [] []))

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