F# 是否继承了 Object 的所有类型？

发布于 2024-10-17 05:45:17 字数 542 浏览 3 评论 0原文

问题很简单并且。尽管答案很明显，但我不得不面对一种奇怪的情况，其中 fsharp 告诉我一些有点奇怪的事情。故事是这样的：

问题是：F# 是否会自动使每种类型继承 Object 类？我想是的，而且我确信这一点，因为如果不是这样的话，将会有很多并发症。

但这是一个事实。我正在写这段代码：

type MyType =
   val myval: int
   override self.ToString = "Hello MyType"

嗯，fsharp 编译器告诉我使用 override 是不正确的，因为他没有找到任何名为 ToString 的方法来重写。我这样编译了这段代码：

type MyType =
   val myval: int
   member self.ToString = "Hello MyType"

一切正常。嗯嗯发生了什么事？？？ FSharp 不是应该从 Object 继承每个对象吗？

原文

The question is simple and. although it is obvious the answer, I had to face a strange situation where the fsharp told me something a bit strange. Here's the story:

The question is: Does F# automatically make every type inherit the Object class? I suppose YES and I am certain of this because there would be so many complications if that were not the case.

But here's a fact.
I was writing this piece of code:

type MyType =
   val myval: int
   override self.ToString = "Hello MyType"

Well, fsharp compiler told me it is not correct using overrid because he does not find any method called ToString to override. I so compiled this code:

type MyType =
   val myval: int
   member self.ToString = "Hello MyType"

And everything worked fine.
mmmmmm What's happening???
Isn't FSharp supposed to inherit every object from Object?

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两个我 2024-10-24 05:45:17

您忘记了括号：

type MyType =
   val myval: int
   override self.ToString() = "Hello MyType"

在省略括号的原始代码中，您试图覆盖属性 ToString。

You forgot parens:

type MyType =
   val myval: int
   override self.ToString() = "Hello MyType"

In your original code with parens omitted you were trying to override property ToString.

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清风无影 2024-10-24 05:45:17

德斯科已经用你的代码片段解释了这个问题。

F# 的行为与 C# 类似，这意味着所有类型都继承自 Object，尽管值类型的行为略有微妙。当您拥有值类型的变量时，不能将其直接视为 Object，而是需要装箱（转换为继承自 Object 的 .NET 引用类型）。

这可能会导致类型推断出现一些令人困惑的错误：

let foo x = 
  match x with 
  | :? System.Random as rnd -> rnd.Next()
  | _ -> 0

当您编写此代码时，您会收到一条错误消息：

错误 FS0008：从类型 'a 到随机的运行时强制或类型测试涉及基于此程序点之前的信息的不确定类型。某些类型不允许运行时类型测试。需要进一步的类型注释。

如果严格意义上从 Object 继承的所有内容，那么 F# 编译器应该能够将任何类型参数转换为 Object，但这是不允许的。问题是值类型需要装箱，因此您需要编写 match box x with ...。

Desco already explained the probelm with your snippet.

F# behaves just like C#, which means that all types inherit from Object, although with a slightly subtle behavior of value types. When you have a variable of a value type, it cannot be directly treated as Object and need to be boxed (to a .NET reference type that inherits from Object).

This can cause some confusing errors with type inference:

let foo x = 
  match x with 
  | :? System.Random as rnd -> rnd.Next()
  | _ -> 0

When you write this, you get an error message:

error FS0008: This runtime coercion or type test from type 'a to Random involves an indeterminate type based on information prior to this program point. Runtime type tests are not allowed on some types. Further type annotations are needed.

If everything inherited from Object in the strict sense, then F# compiler should be able to convert any type parameter to Object, but that's not allowed. The problem is that value types need boxing, so you need to write match box x with ....

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