在类型构造函数中编写泛型仿函数实例？

Question

我正在学习基本类型类,并functor为我的类型编写了自己的实现Test a(行为就像Maybe):

data Test a = Test a | Emp

class FC c a where
  t :: (a -> b) -> c a -> c b

instance FC Test a where
  t f (Test a) = Test (f a) 
  t f (Emp) = Emp

instance FC Maybe a where
  t f (Just a) = Just (f a) 
  t f (Nothing) = Nothing

有可能实现类似的东西:

instance FC c where
  t f (c v) = c (f v)

错误:

Parse error in pattern: c

换句话说,抽象出类型构造函数,替换为c和v,从而创建一个可以应用于具有上下文的任何值的通用实例？

Answer 1

如您所知,c a不是语法上有效的模式.但是,请将您的问题作为功能提案阅读:这将如何运作？并非每个Functor都有一个单元素构造函数可以根据您的模式进行映射.一些例子:

data Pair a = Pair a a  -- more than one element
instance Functor Pair where
    fmap f (Pair x y) = Pair (f x) (f y)

data Proxy a = Proxy  -- no elements
instance Functor Proxy where
    fmap f Proxy = Proxy

newtype Cont r a = Cont { runCont :: (a -> r) -> r }  -- element appears in a double-negative position
instance Functor (Cont r) where
    fmap f (Cont g) = Cont (g . (. f))

无论如何,我认为"通用实例"的想法确实没有意义.该实例是您放置特定于类型的代码的位置.(它必须去某个地方!)

如果您想在编写Functor实例时花费较少的精力,可以使用GHC的DeriveFunctor扩展.

{-# LANGUAGE DeriveFunctor #-}

data Pair a = Pair a a deriving Functor

data Proxy a = Proxy deriving Functor

newtype Cont r a = Cont { runCont :: (a -> r) -> r } deriving Functor