[英]F# Literal/constant can be composed with strings, but not int?
Why is this OK: 为什么这样可以:
let [<Literal>] hi = "hi"
let [<Literal>] bye = "bye"
let [<Literal>] shortMeeting = hi + bye
...but this is not? ......但这不是?
let [<Literal>] me = 1
let [<Literal>] you = 1
let [<Literal>] we = me + you
The third line gives the error: 第三行给出错误:
This is not a valid constant expression
What's up with that? 那是怎么回事?
So the spec / docs are a little unclear, but provide hints. 所以spec / docs有点不清楚,但提供了提示。
From the spec (for F# 3.0): 从规范(对于F#3.0):
A value that has the Literal attribute is subject to the following restrictions: 具有Literal属性的值受以下限制:
It may not be marked mutable or inline. 它可能没有标记为可变或内联。 It may not also have the ThreadStaticor ContextStatic attributes. 它可能也没有ThreadStaticor ContextStatic属性。 The righthand side expression must be a literal constant expression that is made up of either: 右侧表达式必须是由以下任一组成的文字常量表达式:
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. 一个简单的常量表达式,除了(),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。
OR 要么
A reference to another literal 对另一个文字的引用
This seems to suggest that even the combination of strings isn't allowed. 这似乎暗示即使是字符串的组合也是不允许的。
The documentation states that this changed in F# 3.1: 文档说明这在F#3.1中发生了变化:
https://msdn.microsoft.com/en-us/library/dd233193.aspx https://msdn.microsoft.com/en-us/library/dd233193.aspx
As of F# 3.1, you can use the + sign to combine string literals. 从F#3.1开始,您可以使用+号组合字符串文字。 You can also use the bitwise or (|||) operator to combine enum flags. 您还可以使用按位或(|||)运算符来组合枚举标志。 For example, the following code is legal in F# 3.1: 例如,以下代码在F#3.1中是合法的:
Note that integer addition is not on that list 请注意,整数添加不在该列表中
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