处理 F# 中的空值

发布于 2024-10-17 15:37:02 字数 421 浏览 1 评论 0原文

正如您从我提出的大量问题中看到的，我确实对 F# 越来越深入:)

另一个疑问接近我的学习路径：空值。考虑到由于 .NET 框架和 F#（或框架中的任何其他语言）之间的紧密集成而有必要，如何处理它们？

为了简单起见，这里有一段代码：

let myfunc alist =
   try
      List.find (fun x -> true) alist
   with
      | :? KeyNotFoundException as ex -> (* should return null *)

如何在函数中返回 null？
null 关键字没有用，除非被识别（与 nil 不同）。
一般来说，处理 null 返回值时的最佳实践是什么？

原文

As you can see by the mass of questions I'm asking, I'm really getting deeper and deeper into F# :)

Another doubt approaches my learning path: null values. How to handle them considering that it is necessary because of the tight integration between the .NET framework and F# (or any other language in the framework)?

To keep it simple, here's a slice of code:

let myfunc alist =
   try
      List.find (fun x -> true) alist
   with
      | :? KeyNotFoundException as ex -> (* should return null *)

How can I return a null in a function?
The null keyword is useless, unless recognized (not the same for nil).
And, generally speaking, what's the best practice when handling null returned values?

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相对绾红妆 2024-10-24 15:37:02

我不太确定问题是什么。如果完成示例：

open System.Collections.Generic

let myfunc alist =
   try
      List.find (fun x -> true) alist
   with
      | :? KeyNotFoundException as ex -> null

您会发现它编译得很好，并且推断类型 myfunc : 'a list ->; 'a when 'a : null 表示存储在您传入的列表中的类型必须将 null 作为正确值。当使用 C#、VB.NET 等中定义的类型时，F# 完全能够处理空值。

但是，当您不与其他 .NET 语言编写的代码进行互操作时，典型的方法是返回 '一个选项来指示一个值可能存在也可能不存在。然后，您的示例将变为：

let myfunc alist =
   try
      List.find (fun x -> true) alist
      |> Some
   with
      | :? KeyNotFoundException as ex -> None

它将适用于包含任何类型的列表（甚至不具有 null 作为正确值的列表）。当然，在这种情况下，您可以只使用 List.tryFind (fun _ -> true) 来代替。

I'm not quite sure what the question is. If you complete your example:

open System.Collections.Generic

let myfunc alist =
   try
      List.find (fun x -> true) alist
   with
      | :? KeyNotFoundException as ex -> null

you'll find that it compiles just fine, and the inferred type myfunc : 'a list -> 'a when 'a : null indicates that the type stored in the list that you pass in must have null as a proper value. F# is perfectly capable of dealing with null values when using types defined in C#, VB.NET, etc.

However, when you're not interoperating with code written in another .NET language, the typical approach would be to return an 'a option to indicate that a value may or may not be present. Then, your example would become:

let myfunc alist =
   try
      List.find (fun x -> true) alist
      |> Some
   with
      | :? KeyNotFoundException as ex -> None

which will work on lists containing any type (even ones which don't have null as a proper value). Of course, in this case you could just use List.tryFind (fun _ -> true) instead.

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