F#:为什么选项类型与可为空的类型不兼容? [英] F#: Why aren't option types compatible with nullable types?

问题描述

为什么"int选项"之类的选项类型与"Nullable"之类的可空类型不兼容?

Why aren't option types like "int option" compatible with nullable types like "Nullable"?

我认为造成这种差异的原因是一些语义上的原因，但我不知道那是什么.

I assume there is some semantic reason for the difference, but I can't figure what that is.

当值可能存在或不存在时，将使用F#中的选项.选项具有基础类型，并且可以包含该类型的值，也可以不具有值.

An option in F# is used when a value may or may not exist. An option has an underlying type and may either hold a value of that type or it may not have a value.

http://msdn.microsoft.com/zh-cn/library/dd233245％28VS.100％29.aspx

听起来确实像Nullable结构.

That sure sounds like the Nullable structure.

推荐答案

由于System.Nullable<'T>的运行时表示形式选择.

Because of the runtime representation choice for System.Nullable<'T>.

Nullable尝试用null指针表示缺少的值，并通过指向这些值的指针来显示值.

Nullable tries to represent the absent of values by the null pointer, and present values by pointers to those values.

(new System.Nullable<int>() :> obj) = null
|> printfn "%b" // true

(new System.Nullable<int>(1) :> obj).GetType().Name
|> printfn "%s" // Int32

现在考虑字符串.不幸的是，字符串是可以为空的.所以这是有效的:

Now consider strings. Unfortunately, strings are nullable. So this is valid:

null : string

但是现在null运行时值是模棱两可的-它可以表示不存在值或null值.因此，.NET不允许构造System.Nullable<string>.

But now a null runtime value is ambiguous - it can refer to either the absence of a value or a presence of a null value. For this reason, .NET does not allow constructing a System.Nullable<string>.

与此进行对比:

(Some (null : string) :> obj).GetType().Name
|> printfn "%s" // Option`1

话虽如此，一个人可以定义一个双射:

That being said, one can define a bijection:

let optionOfNullable (a : System.Nullable<'T>) = 
    if a.HasValue then
        Some a.Value
    else
        None

let nullableOfOption = function
    | None -> new System.Nullable<_>()
    | Some x -> new System.Nullable<_>(x)

如果观察类型，则这些函数将'T约束为结构，并具有零参数构造函数.因此，也许F#编译器可以通过将.c替换为Option<'T where 'T : struct and 'T : (new : unit -> 'T)>并在必要时插入转换函数来公开.NET函数的接收/返回Nullable<'T>.

If you observe the types, these functions constrain 'T to be a structure and have a zero-argument constructor. So perhaps F# compiler could expose .NET functions receiving/returning Nullable<'T> by substituting it for an Option<'T where 'T : struct and 'T : (new : unit -> 'T)>, and inserting the conversion functions where necessary..

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