Question

我有一个名为Person的代数数据类型如下：

data Person = 
   Person { firstName :: String
          , lastName  :: String
          , height    :: Float
          , age       :: Int
          } deriving (Show, Eq)

我还有一个Person列表，其中包含有关人员的各种信息，现在说我想仅根据Person返回一个特定firstName的所有数据，我该怎么办？那样做？

搜索列表中存储的所有firstName，然后在查询匹配时返回Just Person，如果没有匹配则返回Nothing

Answer 1

您可以使用filter返回包含Person上具有给定条件的所有firstName的列表。

例如：

filter (\x -> firstName x == "John") xs

其中xs包含Person列表。如果没有[]符合条件，这将返回一个空列表（Person）。

Answer 2

很简单，您可以编写自定义查找功能：

lookupBy :: Eq b => (a -> b) -> [a] -> b -> Maybe a
lookupBy _ [] _ = Nothing
lookupBy f (x:xs) key = if f x == key then Just x else lookupBy f xs key

然后你可以用它作为

main = do
    let people = [Person "John" "Doe" 68 30, Person "Jane" "Smith" 63 28]
    print $ lookupBy firstName people "Jane"
    print $ lookupBy lastName  people "Doe"
    print $ lookupBy age       people 29
    print $ lookupBy height    people 63

这会打印

Just (Person {firstName="Jane", lastName="Smith", height=63, age=28})
Just (Person {firstname="John", lastName="Doe", height=58, age=30})
Nothing
Just (Person {firstName="Jane", lastName="Smith", height=63, age=28})

（假设它编译，我没有运行此代码，但它应该工作）。

如果您想选择是获得单个项目还是匹配项目列表，您可以

import Data.Maybe (listToMaybe)

lookupAllBy :: Eq b => (a -> b) -> [a] -> b -> [a]
lookupAllBy f xs key = filter (\x -> f x == key) xs

lookupBy :: Eq b => (a -> b) -> [a] -> b -> Maybe a
lookupBy f xs key = listToMaybe $ lookupAllBy f xs key

根据数据字段haskell查找数据

2 个答案: