如何从给定数据类型的列表中提取字段列表？

Question

如果我已经定义了一个数据类型,比如5个属性.我如何能够创建其中一个属性的列表.

例如:

data Person = Person name fname sexe age height

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

我怎样才能返回一个包含所有名字的列表:( ,Marie,)JohnJessie

Answer 1

您的代码无效Haskell.你可以有

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的列表marie,john以及jessie与

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

现在你可以使用许多内置的功能,如在此列表中操作map和filter:

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

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

现在,如果你将它加载到GHCi中,你可以测试它

> ages db
[25, 26, 27]

有些事情需要注意:

• 如果未构建,则必须声明所有数据类型.
• 用.声明新类型 data TypeName = ConstructorName <FieldTypes>
• 使用声明类型别名 type 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.