无法在F#中创建列表文字 [英] Cannot create list literal in F#

问题描述

我有以下几种类型

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]

我得到一个编译时错误，指出:

And I'm getting a compile-time error that says:

这不是有效的常量表达式或自定义属性值

This is not a valid constant expression or custom attribute value

我真的无法弄清楚这里出了什么问题.

I can't really figure out what's wrong here.

推荐答案

在F#和.NET中，列表不能是文字(在C#/VB.NET中是常数).只有原始值可以，例如string，bool等.F#3.0规范在第10.2.2节中提供了有关可以为字面值或不能为字面值的准则:

In F#, and .NET in general, lists cannot be literals (constant in C#/VB.NET). Only primitive values can, like string, bool, etc. The F# 3.0 specification has the guidelines on what can or cannot be a literal in section 10.2.2:

具有文字"属性的值受以下限制:

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

• 它可能未标记为可变或内联.
• 它可能还没有ThreadStatic或ContextStatic属性.
• 右侧表达式必须是由以下任意一个组成的文字常量表达式:
• 一个简单的常量表达式，但()，本机整数文字，无符号本机整数文字，字节数组文字，BigInteger文字和用户定义的数字文字除外.

• 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.

—或—

• 对另一个文字的引用.

根据要执行的操作，如果在类中使用let绑定，则可以使列表静态.如果在模块中，我将删除Literal属性，因为默认情况下，let绑定是不可变的.

Depending on what you are trying to do, you could make your list static if the let binding is being used in a class. If it is in a module, I'd just remove the Literal attribute since let bindings are immutable by default, anyway.

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