如何在Haskell中建模类层次结构？ [英] How to model class hierarchies in Haskell?

问题描述

我是C＃开发人员。从OO方面来说，我从接口，类和类型层次的角度开始思考。由于Haskell缺乏OO，有时候我发现自己陷入了困境，我无法想出一种用Haskell建模的方法。

如何在Haskell中模拟真实世界中涉及类层次结构的情况，如下所示： http://www.braindelay.com/danielbray/endangered-object-oriented-programming/isHierarchy-4.gif <我们假设以下操作：人类可以说话，狗可以吠叫，并且一个物种的所有成员都可以与之交配如果他们具有相反的性别，则属于同一物种。我会在haskell中这样定义它：

  data Gender = Male |女性获得公式b 
 $ b class物种s其中
 gender :: s  - >性别
 
  - 如果s1和s2可以构想后代
 matable则返回true :: Species a => a  - > a  - > Bool 
 matable s1 s2 =性别s1 / =性别s2 
 
 data Human = Man |女人
数据犬=狗|贱卖
 
实例种类Human其中
性别男人=男性
性别女人=女性
 
实例种类犬类其中
性别狗=男性
性别Bitch =女性
 
树皮Dog =woof
树皮Bitch =哇
 
说话人s =男人说++ s 
说女人s =女人说++ s 
  

现在操作 matable 的类型为 Species s => s - > s - > Bool ， bark 的类型为 Canine - >字符串和 speak 的类型为 Human - >字符串 - >字符串。

我不知道这是否有帮助，但考虑到问题的抽象性，这是我可以提出的最好的

编辑：回复Daniel的评论：

集合的简单层次结构可能如下所示（忽略已存在的类如Foldable和Functor）：

  class Foldable f其中
 fold ::（a  - > b  - > a） - > a  - > f b  - > a 
 
 class可折叠m => Collection m其中
 cmap ::（a  - > b） - > m a  - > m b 
 cfilter ::（a  - > Bool） - > m a  - > m a 
 
类可转换我在哪里
 atIndex :: i a  - > Int  - > 
 $ b实例Foldable []其中
 fold = foldl 
 
实例集合[]其中
 cmap = map 
 cfilter = filter 
 
 instance Indexable []其中
 atIndex =（!!）
 
 sumOfEvenElements ::（Integral a，Collection c）=> c a  - > a 
 sumOfEvenElements c = fold（+）0（cfilter even c）
  

现在sumOfEvenElements需要任何种类的积分集合，并返回该集合中所有偶数元素的总和。

I am a C# developer. Coming from OO side of the world, I start with thinking in terms of interfaces, classes and type hierarchies. Because of lack of OO in Haskell, sometimes I find myself stuck and I cannot think of a way to model certain problems with Haskell.

How to model, in Haskell, real world situations involving class hierarchies such as the one shown here: http://www.braindelay.com/danielbray/endangered-object-oriented-programming/isHierarchy-4.gif

解决方案

Let's assume the following operations: Humans can speak, Dogs can bark, and all members of a species can mate with members of the same species if they have opposite gender. I would define this in haskell like this:

data Gender = Male | Female deriving Eq

class Species s where
    gender :: s -> Gender

-- Returns true if s1 and s2 can conceive offspring
matable :: Species a => a -> a -> Bool
matable s1 s2 = gender s1 /= gender s2

data Human = Man | Woman
data Canine = Dog | Bitch

instance Species Human where
    gender Man = Male
    gender Woman = Female

instance Species Canine where
    gender Dog = Male
    gender Bitch = Female

bark Dog = "woof"
bark Bitch = "wow"

speak Man s = "The man says " ++ s
speak Woman s = "The woman says " ++ s

Now the operation matable has type Species s => s -> s -> Bool, bark has type Canine -> String and speak has type Human -> String -> String.

I don't know whether this helps, but given the rather abstract nature of the question, that's the best I could come up with.

Edit: In response to Daniel's comment:

A simple hierarchy for collections could look like this (ignoring already existing classes like Foldable and Functor):

class Foldable f where
    fold :: (a -> b -> a) -> a -> f b -> a

class Foldable m => Collection m where
    cmap :: (a -> b) -> m a -> m b
    cfilter :: (a -> Bool) -> m a -> m a

class Indexable i where
    atIndex :: i a -> Int -> a

instance Foldable [] where
    fold = foldl

instance Collection [] where
    cmap = map
    cfilter = filter

instance Indexable [] where
    atIndex = (!!)

sumOfEvenElements :: (Integral a, Collection c) => c a -> a
sumOfEvenElements c = fold (+) 0 (cfilter even c)

Now sumOfEvenElements takes any kind of collection of integrals and returns the sum of all even elements of that collection.

这篇关于如何在Haskell中建模类层次结构？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！