如何在F＃中的选项类型的工作 [英] How does the option type work in F#

问题描述

所以，我一直在阅读专家F＃的书由A preSS，建立在一个玩具十岁上下的F＃库，但有一件事我一直没能把握住，这就是选项主要是将其作为一个参考类型

So I've been reading the Expert F# book by Apress, mostly using it as a reference when building a toy-ish F# library, but there's one thing I've failed to grasp and that's the "Option" type.

它是如何工作的，什么是它的现实世界的用法？

How does it work and what is it's real world usage?

推荐答案

选项类型至少相似到可空＆LT; T＆GT; 和在C＃中引用类型。类型的值选项＆LT; T＆GT; 是无这意味着没有encapsuated值，或有些与 T 的特定值。这就像方式的可空＆LT; INT＆GT; 在C＃中的或者的空值，或的都有一个关联 INT - 和方式在C＃字符串值的或者的空引用，或的是指一个String对象。

The option type is at least similar to Nullable<T> and reference types in C#. A value of type Option<T> is either None which means there's no encapsuated value, or Some with a particular value of T. This is just like the way a Nullable<int> in C# is either the null value, or has an associated int - and the way a String value in C# is either a null reference, or refers to a String object.

当您使用的期权价值时，通常指定两个路径 - 一个是那里的的情况下是的关联值，和一个那里的不是的。换句话说，这code：

When you use an option value, you generally specify two paths - one for the case where there is an associated value, and one where there isn't. In other words, this code:

let stringLength (str:Option<string>) =
  match str with
  | Some(v) -> v.Length
  | None -> -1

是类似于：

int StringLength(string str)
{
    if (str != null)
    {
        return str.Length;
    }
    else
    {
        return -1;
    }
}

我相信一般的想法是，强制的你（当然，几乎）处理没有关联的值/对象的情况下让你的code更稳健。

I believe the general idea is that forcing you (well, nearly) to handle the "no associated value/object" case makes your code more robust.

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