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

问题描述

为什么像int option"这样的选项类型与像Nullable"这样的可空类型不兼容?

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

我认为这种差异存在某种语义原因，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，使用 F# 中的选项.一个选项有一个基础类型，可能持有该类型的值，也可能没有值.

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/en-us/library/dd233245%28VS.100%29.aspx

这听起来确实像 Nullable 结构.

That sure sounds like the Nullable structure.

推荐答案

因为 System.Nullable<'T> 的运行时表示选择.

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

Nullable 尝试通过空指针表示值的缺失，并通过指向这些值的指针呈现值.

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，字符串可以为空.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 运行时值是不明确的 - 它可以指不存在值或存在 null 值.因此，.NET 不允许构造 System.Nullable.

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>.

对比:

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

话虽如此，我们可以定义一个双射:

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)

如果您观察类型，这些函数将 'T 约束为一个结构并具有零参数构造函数.因此，也许 F# 编译器可以通过将其替换为 Option<'T where 'T : struct and 'T : (new : unit -> 'T)>，并在必要时插入转换函数..

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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