F＃ - 我可以从一个函数返回一个有区别的联合 [英] F# - Can I return a discriminated union from a function

问题描述

我有以下几种类型：

 类型GoodResource = {
 Id：int; 
 Field1：string} 
 
 
类型ErrorResource = {
 StatusCode：int; 
描述：string} 
  

我有以下歧视联合：

 类型ProcessingResult = 
 |良好的资源
 | ErrorResource的错误
  

然后想拥有一个函数，该函数将具有辨别联合ProcessingResult的返回类型：

 让SampleProcessingFunction值= 
与
匹配值| GoodScenario - > {Id = 123; Field1 =field1data} 
 | _  - > {StatusCode = 456; Description =desc} 
  

是我想要做的事情。编译器发出声明，希望GoodResource成为返回类型。我错过了什么，或者我完全用这种错误的方式去做？

现在， SampleProcessingFunction 为每个分支返回两个不同的类型。

要返回相同的类型，你需要创建一个DU（你所做的），但也明确指定了DU的情况，如下所示：

  let SampleProcessingFunction值= 
与
 |匹配值GoodScenario - >好{Id = 123; Field1 =field1data} 
 | _  - >错误{StatusCode = 456; Description =desc} 
  

您可能会问为什么编译器不能找出正确的大小写自动，但如果您的DU有两种相同类型的情况会发生什么？例如：

 类型GoodOrError = 
 |良好的字符串
 |错误的字符串
  

在下面的例子中，编译器无法确定您的意思：

 让ReturnGoodOrError value = 
与
匹配值| GoodScenario - > 善良
 | _  - > Badness
  

所以你需要使用你想要的构造函数：

 让ReturnGoodOrError value = 
与
匹配值| GoodScenario - >良好的善良
 | _  - >错误Badness
  

I have the following types:

type GoodResource = {
    Id:int;
    Field1:string }


type ErrorResource = {
    StatusCode:int;
    Description:string }

I have the following discriminated union:

type ProcessingResult = 
    | Good of GoodResource
    | Error of ErrorResource

Then want to have a function that will have a return type of the discriminated union ProcessingResult:

let SampleProcessingFunction value =
    match value with
    | "GoodScenario" -> { Id = 123; Field1 = "field1data" }
    | _ -> { StatusCode = 456; Description = "desc" }

Is what I am trying to do possible. The compiler is giving out stating that it expects GoodResource to be the return type. What am I missing or am I completely going about this the wrong way?

解决方案

As it stands, SampleProcessingFunction returns two different types for each branch.

To return the same type, you need to create a DU (which you did) but also specify the case of the DU explicitly, like this:

let SampleProcessingFunction value =
    match value with
    | "GoodScenario" -> Good { Id = 123; Field1 = "field1data" }
    | _ -> Error { StatusCode = 456; Description = "desc" }

You might ask "why can't the compiler figure out the correct case automatically", but what happens if your DU has two cases of the same type? For example:

type GoodOrError = 
    | Good of string
    | Error of string

In the example below, the compiler cannot determine which case you mean:

let ReturnGoodOrError value =
    match value with
    | "GoodScenario" -> "Goodness"
    | _ -> "Badness"

So again you need to use the constructor for the case you want:

let ReturnGoodOrError value =
    match value with
    | "GoodScenario" -> Good "Goodness"
    | _ -> Error "Badness"

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