为什么F#引用不能包含struct? [英] Why F# quotation cannot contains struct?

问题描述

我想定义一个结构，其中包含一些成员方法.我想使用[<ReflectedDefinition>]标记该struct成员方法.但是编译器告诉我这是错误的.

I would like to define a struct, which contains some member methods. And I want to use [<ReflectedDefinition>] to mark that struct member method. But the compiler tells me it is wrong.

首先，请看这段代码:

type Int4 =
    val mutable x : int
    val mutable y : int
    val mutable z : int
    val mutable w : int

    [<ReflectedDefinition>]
    new (x, y, z, w) = { x = x; y = y; z = z; w = w }

    [<ReflectedDefinition>]
    member this.Add(a:int) =
        this.x <- this.x + a
        this.y <- this.y + a
        this.z <- this.z + a
        this.w <- this.w + a

    override this.ToString() = sprintf "(%d,%d,%d,%d)" this.x this.y this.z this.w

它会编译.但是，如果我将类型设为结构，则无法对其进行编译:

It compiles. But if I make the type a struct, it cannot be compiled:

[<Struct>]
type Int4 =
    val mutable x : int
    val mutable y : int
    val mutable z : int
    val mutable w : int

    [<ReflectedDefinition>]
    new (x, y, z, w) = { x = x; y = y; z = z; w = w }

    [<ReflectedDefinition>]
    member this.Add(a:int) = // <----------- here the 'this' report compile error
        this.x <- this.x + a
        this.y <- this.y + a
        this.z <- this.z + a
        this.w <- this.w + a

    override this.ToString() = sprintf "(%d,%d,%d,%d)" this.x this.y this.z this.w

我收到以下错误:

error FS0462: Quotations cannot contain this kind of type

在此代码中指向this.

任何人都知道为什么我不能在struct成员函数上创建引号吗?我猜可能是因为该struct是值类型，所以该指针应该是byref.

Anyone has any idea on why I cannot create quotation on a struct member function? I guess it might because that struct is value type, so this pointer should be byref.

推荐答案

实际上，在F#3.0中观察到的编译器错误为:

In fact, the observed compiler error in F# 3.0 reads:

错误FS1220:ReflectedDefinitionAttribute可能不适用于实例 成员在结构类型上，因为实例成员采用隐式"this" byref参数

error FS1220: ReflectedDefinitionAttribute may not be applied to an instance member on a struct type, because the instance member takes an implicit 'this' byref parameter

观察到的行为的根本原因是F#引号的限制-您不能在引用的代码中使用byref类型.

The root cause of the observed behavior is the limitation of F# quotations - you cannot use byref types in the quoted code.

尽管如此，此限制的结果仅适用于 instance struct成员； static struct成员可以用[<ReflectedDefinition]>属性修饰.

Nevertheless, the consequence of this limitation is applicable only to the instance struct members; static struct members can be decorated with [<ReflectedDefinition]> attribute.

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