功能编程：如何建立关系模型？

Question

我想知道如何在函数式编程中建立关系模型。

以雇员为例：员工可以与0..n同事建立友谊。友谊总是相互的：如果A是B的朋友，那么B是A的朋友。

如何建模？我有3个想法（在下面列出）。他们每个人都有缺点。

第一次尝试：

type Employee =
  { name : string
    friendships : Employee list
  }

// My list is mutable because I want to be able to update employees.
// I suppose the same problems would occur with e. g. Elmish.
let mutable employees : Employee list = [ (* ... *) ]

(* Downsides:
   Friendships are copies. Changes to an employee would have to be propagated manually.
*)

第二次尝试：

type EmployeeId = int

type Employee =
  { id : EmployeeId
    name : string
    friendships : EmployeeId list
  }

let mutable employees : Employee list = [ (* ... *) ]

(* Downsides:
   Friendships are not modeled as mutual.
   When the friendship of an employee changes,
   the friend's friendships don't change accordingly.
*)

第三次尝试：

type EmployeeId = int

type Friendships = list<EmployeeId * EmployeeId>

type Employee =
  { id : EmployeeId
    name : string
  }

let mutable employees : Employee list = [ (* ... *) ]

(* Downsides:
   After deleting an employee from the list,
   employeeFriendships contain invalid "references"
   to the deleted Employee.
*)

这可以做得更好吗？谢谢。

Answer 1

使用let rec / and可能是最直接的方法。您需要通过将friendships更改为IEnumerable来引入懒惰。

type Employee =
    { 
        name : string
        friendships : seq<Employee>
    }

let rec peter = {name="Peter"; friendships=seq {yield paul; yield mary}}
and paul = {name="Paul"; friendships=seq {yield peter; yield mary}}
and mary = {name="Mary"; friendships=seq {yield peter; yield paul}}

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