创建实例时设置记录属性

Question

我有一个记录类型

type Record = { Start: DateTime; End: DateTime; Duration: TimeSpan }
let record = { Start = DateTime.Now; End = DateTime(2021,12,1) }

此代码将无法编译，因为没有为 field 指定分配Duration。

是否可以Duration在类型定义中进行计算而不是赋值？

Answer 1

解决了：

type Record = { Start: DateTime; End: DateTime; }
with
  member this.Duration = this.End - this.Start

let record = { Start = DateTime.Now; End = DateTime(2021,12,1) }

Answer 2

如果你不想Duration每次调用时都重新计算，你可以这样做：

type Record = private { Start: DateTime; End: DateTime; Duration: TimeSpan }

let create startTime endTime =
    {
        Start = startTime
        End = endTime
        Duration = endTime - startTime
    }

let record = create DateTime.Now (DateTime(2021,12,1))

请注意，我已经创建了这些字段private，因此用户将被迫调用create。这种方法的唯一缺点是用户根本无法再访问这些字段，因此您可能需要为它们提供单独的访问器：

type Record = private { _Start: DateTime; _End: DateTime; _Duration: TimeSpan } with
    member this.Start = this._Start
    member this.End = this._End
    member this.Duration = this._Duration

let create startTime endTime =
    {
        _Start = startTime
        _End = endTime
        _Duration = endTime - startTime
    }

let record = create DateTime.Now (DateTime(2021,12,1))