为具有更高种类参数的数据自动生成映射函数

Automatically generate mapping function for data with higher-kinded parameter

考虑数据类型

data Foo f = Foo {fooInt :: f Int, fooBool :: f Bool}

我想要一个函数 mapFoo :: (forall a. f a -> g a) -> Foo f -> Foo g。我的选择:

如果有一种通用的方式来编写 mapFoo 而无需使用模板 Haskell,我很想知道它!谢谢。

The rank2classes package can derive this 给你。

{-# LANGUAGE TemplateHaskell #-}

import Rank2.TH (deriveFunctor)

data Foo f = Foo {fooInt :: f Int, fooBool :: f Bool}

$(deriveFunctor ''Foo)

现在你可以写 mapFoo = Rank2.(<$>).

编辑:哦,我应该明确这是一个 手动 方法 - 它是一个指向具有许多有用功能和类型 类 的包的指针但是afaik 没有 TH 来生成你想要的东西。欢迎提出请求,我敢肯定。

parameterized-utils package provides a rich set of higher rank classes. For your needs there's FunctorF:

-- | A parameterized type that is a function on all instances.
class FunctorF m where
  fmapF :: (forall x . f x -> g x) -> m f -> m g

这些实例可能就是您所期望的:

{-# LANGUAGE RankNTypes #-}
import Data.Parameterized.TraversableF

data Foo f = Foo {fooInt :: f Int, fooBool :: f Bool}

instance FunctorF Foo where
  fmapF op (Foo a b) = Foo (op a) (op b)

这里是基于 GHC.Generics 的实现,如果你仍然不想使用 TemplateHaskell:

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}

import GHC.Generics

data Foo f = Foo {
    fooInt :: f Int,
    fooBool :: f Bool,
    fooString :: f String
    } deriving (Generic)


class Functor2 p q f where
    fmap2 :: (forall a. p a -> q a) -> f p -> f q

instance (Generic (f p), Generic (f q), GFunctor2 p q (Rep (f p)) (Rep (f q))) => Functor2 p q f where
    fmap2 f = to . (gfmap2 f) . from 

class GFunctor2 p q f g where
    gfmap2 :: (forall a. p a -> q a) -> f x -> g x

instance (GFunctor2 p q a b) => GFunctor2 p q (D1 m1 (C1 m2 a)) (D1 m1 (C1 m2 b)) where
    gfmap2 f (M1 (M1 a)) = M1 (M1 (gfmap2 f a))

instance (GFunctor2 p q a c, GFunctor2 p q b d) => GFunctor2 p q (a :*: b) (c :*: d) where
    gfmap2 f (a :*: b) = gfmap2 f a :*: gfmap2 f b

instance GFunctor2 p q (S1 m1 (Rec0 (p a))) (S1 m1 (Rec0 (q a))) where
   gfmap2 f (M1 (K1 g)) = M1 (K1 (f g))   


-- Tests
foo = Foo (Just 1) (Just True) (Just "foo")

test1 = fmap2 (\(Just a) -> [a]) foo   

test2 = fmap2 (\[a] -> Left "Oops") test1

我不确定是否可以避免 MultiParamTypeClasses 使 class Functor2 与定义的 rank2classes 相同。