[英]F# Create custom attribute to expression
In F#, how can I create a custom attribute to apply on expressions? 在F#中,如何创建自定义属性以应用于表达式? I looked for resources everywhere, but I didn't find nothing.
我到处寻找资源,但我一无所获。
For exemple The attribute [<Entrypoint>]
can be applied to some expression, and by consequence the compiler can infer that expression should be of type array string -> int
. 例如,属性
[<Entrypoint>]
可以应用于某个表达式,因此编译器可以推断该表达式应该是类型array string -> int
。
How can I create a custom attribute to work simillary ? 如何创建自定义属性以使用simillary?
To create a custom attribute, simply declare a class that inherits from System.Attribute
: 要创建自定义属性,只需声明一个继承自
System.Attribute
的类:
type MyAttribute() = inherit System.Attribute()
[<My>]
let f x = x+1
As you can see, the suffix "Attribute" can be omitted when applying the attribute to code units. 如您所见,将属性应用于代码单元时可以省略后缀“Attribute”。 Optionally, you can give your attribute parameters or properties:
(可选)您可以提供属性参数或属性:
type MyAttribute( x: string ) =
inherit System.Attribute()
member val Y: int = 0 with get, set
[<My("abc", Y=42)>]
let f x = x+1
At runtime, you can inspect types, methods, and other code units to see which attributes are applied to them, and to retrieve their data: 在运行时,您可以检查类型,方法和其他代码单元,以查看应用于哪些属性,以及检索其数据:
[<My("abc", Y=42)>]
type SomeType = A of string
for a in typeof<SomeType>.GetCustomAttributes( typeof<MyAttribute>, true ) do
let my = a :?> MyAttribute
printfn "My.Y=%d" my.Y
// Output:
> My.Y=42
Here is a tutorial explaining custom attributes in more detail. 这是一个更详细地解释自定义属性的教程 。
However , you cannot use custom attributes to enforce compile-time behavior. 但是 ,您无法使用自定义属性来强制执行编译时行为。 The
EntryPointAttribute
is special - that is, the F# compiler knows about its existence and gives it special treatment. EntryPointAttribute
是特殊的 - 也就是说,F#编译器知道它的存在并给予特殊处理。 There are some other special attributes in F# - for example, NoComparisonAttribute
, CompilationRepresentationAttribute
, etc., - but you cannot tell the compiler to give special treatment to attributes that you yourself created. F#中还有一些其他特殊属性 - 例如,
NoComparisonAttribute
, CompilationRepresentationAttribute
等,但您无法告诉编译器对您自己创建的属性进行特殊处理。
If you describe your larger goal (ie what you're trying to achieve), I'm sure we'll be able to find a better solution. 如果你描述了你更大的目标(即你想要实现的目标),我相信我们能够找到更好的解决方案。
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