如何从给定数据类型的列表中提取字段列表？ [英] How to extract a list of fields from a list of a given data type?

问题描述

如果我已经定义了一个数据类型，比方说5个属性。我如何能够创建一个属性列表。

If I have defined a datatype with, let's say, 5 attributes. How would I be able to create a list of one of the attributes.

例如：

data Person = Person name fname sexe age height

Marie, John, Jessie :: Person
Marie = Person "Marie" _ _ _
John = Person "John" _ _ _
Jessie = Person "Jessie" _ _ _

我可以返回一个包含所有名称的列表：（ Marie ， John ， Jessie code>）

How can I return a list containing all the names : (Marie, John, Jessie)

推荐答案

您的代码不是有效的Haskell。你可以有

Your code isn't valid Haskell. You could have

data Person = Person FName LName Sexe Age Height
type FName = String
type LName = String
data Sexe = Male | Female
type Age = Int
type Height = Float

marie, john, jessie :: Person
marie = Person "Marie" "Lastname1" Female 25 165
john = Person "John" "Lastname2" Male 26 180
jessie = Person "Jessie" "Lastname3" Female 27 170

然后你可以创建一个包含三个值 marie ， john 的列表，和 jessie with

Then you could create a list containing the three values marie, john, and jessie with

db :: [Person]
db = [marie, john, jessie]

现在您可以使用许多内置函数如 map 和过滤器：

Now you can operate on this list using many built in functions like map and filter:

getAge :: Person -> Age
getAge (Person _ _ _ age _) = age

ages :: [Person] -> [Age]
ages people = map getAge people

现在，如果您将此加载到GHCi中可以测试它为

Now if you load this into GHCi you can test this as

> ages db
[25, 26, 27]

有些事情要注意：

• 所有数据类型必须声明，如果它们不是内置的。


• 用<$声明新类型c $ c> data TypeName = ConstructorName< FieldTypes>


• 用声明别名类型AliasName = ExistingType


• 类型名称和构造函数必须以大写字母开头


• 值名称必须以小写字母开头

• All data types must be declared if they aren't build in.
• Declare new types with data TypeName = ConstructorName <FieldTypes>
• Declare type aliases with type AliasName = ExistingType
• Type names and constructors must start with a capital letter
• Value names must start with a lower case letter.

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