F# Literal/constant 可以用字符串组合，不能用int？

Question

为什么这样可以：

let [<Literal>] hi = "hi"
let [<Literal>] bye = "bye"
let [<Literal>] shortMeeting = hi + bye

...但这不是吗？

let [<Literal>] me = 1
let [<Literal>] you = 1
let [<Literal>] we = me + you

第三行报错：

This is not a valid constant expression

这是怎么回事？

Answer 1

所以规范/文档有点不清楚，但提供提示。

来自规范（针对 F# 3.0）：

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 ThreadStaticor ContextStatic attributes. The righthand 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

这似乎表明即使是字符串组合也是不允许的。

文档指出这在 F# 3.1 中发生了变化：

https://msdn.microsoft.com/en-us/library/dd233193.aspx

As of F# 3.1, you can use the + sign to combine string literals. You can also use the bitwise or (|||) operator to combine enum flags. For example, the following code is legal in F# 3.1:

请注意，整数加法不在该列表中

F# Literal/constant 可以用字符串组合，不能用int？

F# Literal/constant can be composed with strings, but not int?

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