在可区分联合中使用 F# 中的 and 关键字
我今天面临以下 DU 声明：
type Grammar = Definition list
and Definition = Def of string * Expression
and Range =
| Char of char
| Range of char * char
为什么在这里使用关键字 and 而不是 type？
I was faced today with the following DUs declarations:
type Grammar = Definition list
and Definition = Def of string * Expression
and Range =
| Char of char
| Range of char * char
Why would one use the keyword and instead of type, here?
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Grammar和Definition的定义需要and才能正确编译。首先列出Grammar类型，但取决于稍后定义的Definition类型。为了正确编译，它必须与and链接，告诉 F# 编译器类型定义是依赖/相关的。没有理由以这种方式声明
Range，而应该使用type来声明The
andis needed for the definitions ofGrammarandDefinitionto compile correctly. TheGrammartype is listed first but depends on the typeDefinitionwhich is defined later. In order to compile properly it must be linked withandwhich tells the F# compiler the type definitions are dependent / related.There is no reason for
Rangeto be declared in such a way and should be declared withtype它用于创建相互相关的类型。通常在 F# 中，您需要在使用每种类型之前对其进行转发声明 - 但这并不总是可行，例如，当您需要引入对两种或多种类型的循环依赖时。
在您的示例中，如果您使用
type而不是and定义Definition，您将无法编译Grammar 的定义，除非您更改了它们的定义顺序。您发布的代码示例并不完全是一个好的代码示例，因为其中不需要相互关系 - 您可以更改顺序。 （除非有更多的类型进一步定义，这取决于上面的内容）。
It's used to create mutually related types. Usually in F#, you need to forward declare each type before you use it - but this isn't always possible, for example when you need to introduce a cyclic dependency on two or more types.
In your example, if you defined
Definitionwithtyperather thanand, you wouldn't be able to compile the definition ofGrammar, unless you switched the order in which they're defined.The code example you've posted isn't exactly a good one, because the mutual relation isn't necessary in it - you can change the order. (Unless there were some more types defined further down which depended on the above).