区分联盟内的匿名记录类型

Question

F#教程包括以下代码段:

/// A record for a person's first and last name
type Person = {     
    First : string
    Last  : string
}

/// define a discriminated union of 3 different kinds of employees
type Employee = 
    | Engineer  of Person
    | Manager   of Person * list<Employee>            // manager has list of reports
    | Executive of Person * list<Employee> * Employee // executive also has an assistant

经理和执行官被描述为元组的事实冒犯了我的感情(我很容易被冒犯).它让我觉得不那么富有表现力.我试着按如下方式修改它们:

/// define a discriminated union of 3 different kinds of employees
type Employee = 
    | Engineer  of Person
    | Manager   of { Name: Person; Reports: Employee list }            // manager has list of reports
    | Executive of { Name: Person; Reports: Employee list; Assistant: Employee}   // executive also has an assistant

遗憾的是,Manager和Executive的定义现在给出了一个错误:"不推荐使用此构造;请考虑使用单独的记录类型." 好吧,看起来很公平,我们称之为ManagerType.但是等待...... ManagerType指的是Employee(对于Reports部分),Employee指的是ManagerType(对于Manager选项).

这里有解决方案吗？两个数据结构是否不可能相互定义？

Answer 1

您可以使用and以下命

    type Employee = 
    | Engineer  of Person
    | Manager   of Manager            // manager has list of reports
    | Executive of Executive
    and Manager = { Name: Person; Reports: Employee list }
    and Executive = { Name: Person; Reports: Employee list; Assistant: Employee }

Answer 2

如果你正在使用F#v3.1,你可以使用命名联合字段^[MSDN](并保护你敏感的感觉):

type Employee = 
    | Engineer  of Person
    | Manager   of Name: Person * Reports: Employee list
    | Executive of Name: Person * Reports: Employee list * Assistant: Employee

Answer 3

相互递归类型声明为type ... = ... and ... = ...:

type Employee = 
    | Engineer  of Person
    | Manager   of Manager 
    | Executive of Executive
and Manager = { Name: Person; Reports: Employee list }
and Executive = { Name: Person; Reports: Employee list; Assistant: Employee}

Answer 4

从 FSharp 4.6 开始，您可以将匿名记录与联合一起使用。因此，您的示例可以这样定义：

type Person = {     
    First : string
    Last  : string
}

type Employee =
    | Engineer of Person
    | Manager of {| Name: Person; Reports: Employee list |}
    | Executive of {| Name: Person; Reports: Employee list; Assistant: Employee |}