f# - F# 中的强类型 ID？

Question

我的应用程序中有两种实体:客户和产品。它们每个都在数据库级别由 UUID 标识。

在我的 F# 代码中，这可以表示为 System.Guid .

为了可读性，我添加了一些这样的类型:

open System

type CustomerId = Guid

type ProductId = Guid

但是，这并不妨碍我使用 ProductId作为 CustomerId反之亦然。

我想出了一个包装器的想法来防止这种情况:

open System

[<Struct>]
type ProductId = 
  {
    Product : Guid
  }

[<Struct>]
type CustomerId = 
  {
    Customer : Guid
  }

这使得初始化更加冗长，而且可能不那么直观:

let productId = { Product = Guid.NewGuid () }

但它增加了类型安全性:

// let customerId : CustomerId = productId // Type error

我想知道还有哪些其他方法。

Answer 1

您可以使用 single-case union类型:

open System

[<Struct>]
type ProductId = ProductId of Guid

[<Struct>]
type CustomerId = CustomerId of Guid

let productId = ProductId (Guid.NewGuid())

通常我们会直接向类型添加一些方便的辅助方法/属性:

[<Struct>]
type ProductId = private ProductId of Guid with
    static member Create () = ProductId (Guid.NewGuid())
    member this.Value = let (ProductId i) = this in i

[<Struct>]
type CustomerId = private CustomerId of Guid with
    static member Create () = CustomerId (Guid.NewGuid())
    member this.Value = let (CustomerId i) = this in i

let productId = ProductId.Create ()
productId.Value |> printfn "%A"