过滤一个haskell数据类型 [英] Filter a haskell data type

问题描述

这是我到目前为止的代码：

  arcticMonkeysAreTheBest =回顾Arctic Monkeys
 5 
 arcticMonkeys2012 
18/08/2013​​
België
节
 [我敢打赌，你在舞池上看起来不错，当太阳下山时， 仍然带你回家] 
 
 arcticMonkeysRock =回顾Artic Monkeys
 5 
 arcticMonkeys2013 
17/08/2013​​
België 
 Zaal 
 [RU Mine？，Arabella，你为什么只在你高的时候打电话给我？] 
 
 reviews = [arcticMonkeysAreTheBest，arcticMonkeysRock ] 
  

现在我的问题是：如何过滤 Location = Zaal 例如？

也可以用两个标准过滤吗？例如， artist = Arctic Monkeys 和其中 Location = Zaal ？

解决方案

我可能会将您的评论类型定义为

  data Review =查看
 {reviewBand :: String 
，reviewStars :: Integer 
，reviewTour :: Tour 
，reviewData :: String 
，reviewCountry :: String 
 ，reviewLocation :: Location 
，reviewSongs :: [String] 
}派生（Eq，Ord，Show）
  

和您的位置类型为

 数据位置=节日| Zaal派生（Eq，Ord，Show）
  

然后你就可以轻松地做到了

 >>过滤器（\review-> reviewLocation review == Zaal）评论
  

或更简洁 p>

 >>过滤器（（== Zaal）。reviewLocation）评论
  

$ b 编辑 p>

如果你想把它作为一个函数，就像定义

  filterByLocation :: Location  - > [评论]  - > [Review] 
 filterByLocation location = filter（\r  - > reviewLocation r == location）
  

This is my code I have so far:

    arcticMonkeysAreTheBest = Review    "Arctic Monkeys"
                                5
                                arcticMonkeys2012
                                "18/08/2013"
                                "België"
                                Festival
                                ["I bet you look good on the dancefloor", "When the sun goes down", "Still take you home"]

    arcticMonkeysRock = Review  "Artic Monkeys"
                        5
                        arcticMonkeys2013
                        "17/08/2013"
                        "België"
                        Zaal
                        ["R U Mine?", "Arabella", "Why'd you only call me when you're high?"]

    reviews = [arcticMonkeysAreTheBest, arcticMonkeysRock]

Now my question is: How can I filter the reviews with Location = Zaal for example?

Is it also possible to filter with two criteria? For example where artist= Arctic Monkeys and where Location = Zaal?

解决方案

I would probably define your review type to be

data Review = Review
  { reviewBand     :: String
  , reviewStars    :: Integer
  , reviewTour     :: Tour
  , reviewData     :: String
  , reviewCountry  :: String
  , reviewLocation :: Location
  , reviewSongs    :: [String]
  } deriving (Eq,Ord,Show)

and your Location type to be

data Location = Festival | Zaal deriving (Eq,Ord,Show)

You can then easily do

>> filter (\review -> reviewLocation review == Zaal) reviews

or even more concisely

>> filter ((== Zaal) . reviewLocation) reviews

Edit

If you want this as a function, it is as simple as defining

filterByLocation :: Location -> [Review] -> [Review]
filterByLocation location = filter (\r -> reviewLocation r == location)

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