如何在 F# 中为 T[] 定义类型扩展? [英] How to define a type extension for T[] in F#?

问题描述

在 C# 中，我可以为 T 类型的泛型数组定义一个扩展方法，如下所示:

In C#, I can define an extension method for a generic array of type T like this:

public static T GetOrDefault<T>(this T[] arr, int n)
{
    if (arr.Length > n)
    {
        return arr[n];
    }

    return default(T);
}

但对于我的生活，我不知道如何在 F# 中做同样的事情！我试过 type 'a array with, type array<'a>with 和 type 'a[] with 编译器对它们中的任何一个都不满意.

but for the life of me I can't figure out how to do the same in F#! I tried type 'a array with, type array<'a> with and type 'a[] with and the compiler wasn't happy with any of them.

谁能告诉我在 F# 中这样做的权利是什么?

Can anyone tell me what's the right to do this in F#?

当然，我可以通过覆盖 Array 模块并为此轻松添加一个函数来做到这一点，但我真的很想知道如何将其作为扩展方法来实现！

Sure, I can do this by overshadowing the Array module and add a function for that easily enough, but I really want to know how to do it as an extension method!

推荐答案

您必须使用反引号"来编写数组类型 - 像这样:

You have to write the array type using 'backtick marks' - like this:

type 'a ``[]`` with
  member x.GetOrDefault(n) = 
    if x.Length > n then x.[n]
    else Unchecked.defaultof<'a>

let arr = [|1; 2; 3|]
arr.GetOrDefault(1) //2
arr.GetOrDefault(4) //0

编辑:语法type ``[]``<'a>with ... 似乎也被允许.在 F# 源代码 (prim-types-prelude.fs) 中，您可以找到以下定义:

Edit: The syntax type ``[]``<'a> with ... seems to be allowed as well. In the F# source (prim-types-prelude.fs) you can find the following definition:

type ``[]``<'T> = (# "!0[]" #)

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