haskell - 在 haskell 中使用泛型列出构造函数名称

Question

我知道如何以下列方式列出构造函数名称:

{-# LANGUAGE DeriveGeneric        #-}
{-# LANGUAGE FlexibleContexts     #-}
{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE PolyKinds            #-}
{-# LANGUAGE ScopedTypeVariables  #-}
{-# LANGUAGE TypeApplications     #-}
{-# LANGUAGE TypeOperators        #-}

module Generics where     

import           Data.Proxy          
import           GHC.Generics        

class Names' f where
  names' :: Proxy f -> [String]
instance (Names' f) => Names' (M1 D t f) where
  names' _ = names' (Proxy :: Proxy f)
instance (Names' f, Names' g) => Names' (f :+: g) where
  names' _ = (names' (Proxy :: Proxy f) ++ (names' (Proxy :: Proxy g)))
instance (Constructor c) => Names' (C1 c f) where
  names' _ = [conName (undefined :: C1 c f g)]

data Test = Foo | Bar Int | Baz { x :: Int } deriving (Generic)

main :: IO ()
main = do
  print $ names' (Proxy :: Proxy (Rep Test))
--> ["Foo", "Bar", "Baz"]

但在那种情况下，我们需要一个 Proxy (Rep Test) .如何用给定的 Proxy Test 做同样的事情?

我找到了一个 example如何获得记录的第一个选择器，但不知道如何解决我的问题。请帮忙。

Answer 1

简单:

{-# LANGUAGE ScopedTypeVariables, FlexibleContexts #-}
names :: forall t. (Generic t, Names' (Rep t)) => Proxy t -> [String]
names _ = names' (Proxy :: Proxy (Rep t))

之所以有效是因为 Proxy s 无关紧要:只要您可以命名类型，您就可以构建 Proxy为了它。 (旁注:除非您使用 GHC <8，否则您应该使用 AllowAmbiguousTypes 和 TypeApplications 扩展 Proxy 。在 GHC >=8 上，您唯一需要 Proxy 之类的东西就在更高级别的上下文。)

你也可以说

{-# LANGUAGE ScopedTypeVariables, FlexibleContexts, UndecidableInstances #-}
class (Generic t, Names' (Rep t)) => Names t where
  names :: Proxy t -> [String]
instance (Generic t, Names' (Rep t)) => Names t where
  names _ = names' (Proxy :: Proxy (Rep t))

获得一个很好的约束同义词 Names t为 (Generic t, Names' (Rep t)) .