使用记录 Haskell 的泛型派生

Question

我基本上是想看看我是否可以在 Haskell 中模拟 ORM 框架，这样如果用户想要制作数据库模型，他们会做这样的事情

data Car = Car {
        company :: String, 
        model :: String, 
        year :: Int
        } deriving (Model)

表格是“汽车”，列是公司、型号、年份

要在 Haskell 中执行此操作，您必须结合使用类和泛型，这就是我遇到的问题。使用本教程 (http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/generic-programming.html)，我想出了这个（基本上是复制和重命名，所以我可以让代码正常工作）

{-# LANGUAGE DeriveGeneric, TypeOperators, TypeSynonymInstances, FlexibleInstances #-}

module Main where
import GHC.Generics

class SModel b where
        s_new :: b -> IO()

instance SModel Int where
        s_new s = putStrLn s++":Int"

instance SModel Integer where
        s_new s = putStrLn s++":Integer"

instance SModel String where
        s_new s = putStrLn s++":String"    

class Model m where
        new :: m a -> IO()

instance Model U1 where
        new U1 = putStrLn "unit"

instance (Model a, Model b) => Model (a :*: b) where
        new (a :*: b) = do
                new a
                new b

instance (Model a, Model b) => Model (a :+: b) where
        new (L1 x) = new x
        new (R1 x) = new x

instance (Model a) => Model (M1 i c a) where
        new (M1 x) = new x

instance (SModel a) => Model (K1 i a) where
        new (K1 x) = s_new x

data Car = Car {
        company :: String, 
        model :: String, 
        year :: Int
        } deriving (Model)

以上代码会报错

Cannot derive well-kinded instance of form `Model (Car ...)'
      Class `Model' expects an argument of kind `* -> *'
    In the data declaration for `Car'

我有点卡在这一点上，我相信我已经实例化了所有必需的泛型类型来覆盖记录

Answer 1

正如kosmikus 在他的评论中所说，你不能直接推导出Model。首先，您需要一个为Model 提供通用默认值的“前端”类，如下所示：

class FModel a where
    fnew :: a -> IO()

    default new :: (Generic a, Model (Rep a)) => a -> IO()
    fnew = new . from

那么你可以这样做：

Car = ... deriving Generic
instance FModel Car

你有想要的实例。