无法在 F# 中创建列表文字

Question

我有以下类型

type StatusCode = 
    | OK          = 200
    | NoContent   = 204
    | MovedTemp   = 301
    | MovedPerm   = 302
    | SeeOther    = 303
    | NotModified = 304
    | NotFound    = 404
    | ServerError = 500

[<Literal>]
let NoBodyAllowedStatusCodes = [StatusCode.NoContent; StatusCode.NotModified]

我收到一个编译时错误：

This is not a valid constant expression or custom attribute value

我真的搞不懂这里出了什么问题。

Answer 1

在 F# 和一般的 .NET 中，列表不能是文字（在 C#/VB.NET 中为常量）。只有原始值可以，如 string、bool 等。F# 3.0 规范在第 10.2.2 节中有关于什么可以或不能是文字的指南：

A value that has the Literal attribute is subject to the following restrictions:

It may not be marked mutable or inline.

It may not also have the ThreadStatic or ContextStatic attributes.

The right-hand side expression must be a literal constant expression that is made up of either:

A simple constant expression, with the exception of (), native integer literals, unsigned native integer literals, byte array literals, BigInteger literals, and user-defined numeric literals.

—OR—

A reference to another literal.

根据您尝试执行的操作，如果 class 中使用了 let 绑定，您可以将列表设为静态。如果它在模块中，我将删除 Literal 属性，因为 let 绑定默认情况下是不可变的。

无法在 F# 中创建列表文字

Cannot create list literal in F#

f#

compile-time-constant

constant-expression