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我正在寻找有关以下代码的一些重构/最佳实践建议。我想尽量避免扩展，同时为互斥的不同“版本”维护单独的模块。我目前的解决方案是使用一个类并使用存在量化为每个CountrySets.

如果我使用 OOP，这对我来说很容易完成，但我还不能考虑“功能性”。

谢谢你的时间。

省份.hs

{-# LANGUAGE ExistentialQuantification, RankNTypes  #-}
module Province where

  class Country a where
    --some stuff that is not important

  data Power = 
    forall a. (Show a, Eq a, Country a) => Power a |
    Netural |
    Water

  data Unit = Unit {
    power  :: forall a. (Show a, Eq a, Country a) => a, 
    piece :: Piece

  data Piece = Tank | Plane

  data Province = Province {
    power  :: Power,
    provName :: String
  } deriving (Eq)

  instance Show Power where
    show (Power b) = "Power " ++ show b
    show (Netural) = "Netural"
    show (Water) = "Water"

  instance Eq Power where
    (==) a b = Prelude.show a == Prelude.show b

版本1.hs

import Province

  data CountrySet1 =
    Country11 |
      Country12 
    deriving (Eq, Show)
  instance Country CountrySet1 where

  provs = 
    one1:one2:[]

  one1 = Province (Power Country11) "Place11"
  one2 = Province (Power Country12) "Place12"

版本2.hs

import Province

  data CountrySet2 =
    Country21 |
    Country22 
      deriving (Eq, Show)
  instance Country CountrySet2 where

  provs = 
    two1:two2:[]

  two1 = Province (Power Country11) "Place21"
  two2 = Province (Power Country12) "Place22"
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1 回答 1

7

您不必在数据类型中放置类约束。您可以改为在a变量上参数化您的数据类型，以便您可以将约束放在类型类实例本身上，如下所示：

-- Note that I added a type variable to "Power"
data Power a = Power a | Neutral | Water

instance (Show a) => Show (Power a) where ...

instance (Eq a) => Eq (Power a) where ...

...或者你可以做大多数人做和使用的事情deriving：

data Power a = Power a | Neutral | Water deriving (Eq, Show)

这会生成与您编写的完全相同的实例（除了Eq一个将比您编写的更有效）。无需扩展！

那么如果你想a成为一个特定的类型，你就这么说吧！

-- Version1.hs
myValue1 :: Power CountrySet1
myValue1 = ...


-- Version2.hs
myValue2 :: Power CountrySet2
myValue2 = ...

然后它们是完全兼容的，并且两种实现可以相互共存。

于 2012-08-06T23:07:25.677 回答