[< Literal>]与F#中的其他常数有何不同 [英] how does [<Literal>] differ from other constants in F#

问题描述

我对Literal关键字以及为什么在F#中有必要感到困惑.

I am a bit confused by the Literal keyword and why it is necessary in F#.

在阅读文档时，听起来像[<Literal>]用于定义常量，但是我有点困惑这个常量与F#中所有其他常量的区别.

Reading the docs, it sounds to me that [<Literal>] is used to define a constant, however I am a bit confused how this constant differs from all other constants in F#..

旨在为常量的值可以用 文字属性.此属性的作用是使 被编译为常量.

Values that are intended to be constants can be marked with the Literal attribute. This attribute has the effect of causing a value to be compiled as a constant.

当我想到一个常数时，我想到的是不可变的....

When I think of a constant, I think of something which is immutable....

let x = "a" + "b" //this is a immutable value, its value is constant
[<Literal>]
let y = "a" + "b" //this is also a immutable value, but why is this a special constant?

是因为对正常" F#值的计算是惰性的，而对[<Literal>]的计算却不是惰性的.是编译为常量"的意思.还是还有其他东西?

Is it because the 'normal' F# values are evaluated lazily and the [<Literal>] is not evaluated lazily..? is that what they mean with 'compiled as constant'..? or is there something else to it?

推荐答案

在您的示例中，x是在运行时分配的一个不可变值(但不是懒惰地求值)，而y是在编译时分配的.例如，

In your example, x is an immutable value that is assigned during runtime (but NOT evaluated lazily), whereas y is assigned during compiletime. As an example,

let myDLL = "foo.dll"

[<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>]
extern void HelloWorld()

将不起作用，因为DllImport是一个属性，在编译过程中需要知道myDLL的值.但是，这将起作用:

will not work, because DllImport is an attribute and needs to know the value of myDLL during compilation. However, this will work:

[<Literal>]
let myDLL = "foo.dll"

[<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>]
extern void HelloWorld()

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