使用 typeof<_>;处于活跃模式

发布于 2024-11-25 09:07:23 字数 844 浏览 0 评论 0原文

给定以下人为的活动模式：

let (|TypeDef|_|) (typeDef:Type) (value:obj) =
  if obj.ReferenceEquals(value, null) then None
  else
    let typ = value.GetType()
    if typ.IsGenericType && typ.GetGenericTypeDefinition() = typeDef then Some(typ.GetGenericArguments())
    else None

以下：

let dict = System.Collections.Generic.Dictionary<string,obj>()
match dict with
| TypeDef typedefof<Dictionary<_,_>> typeArgs -> printfn "%A" typeArgs
| _ -> ()

给出错误：

模式匹配中的意外类型应用。预期为“->”或其他令牌。

但这是有效的：

let typ = typedefof<Dictionary<_,_>>
match dict with
| TypeDef typ typeArgs -> printfn "%A" typeArgs
| _ -> ()

为什么这里不允许 typedefof （或 typeof）？

原文

Given the following contrived active pattern:

let (|TypeDef|_|) (typeDef:Type) (value:obj) =
  if obj.ReferenceEquals(value, null) then None
  else
    let typ = value.GetType()
    if typ.IsGenericType && typ.GetGenericTypeDefinition() = typeDef then Some(typ.GetGenericArguments())
    else None

The following:

let dict = System.Collections.Generic.Dictionary<string,obj>()
match dict with
| TypeDef typedefof<Dictionary<_,_>> typeArgs -> printfn "%A" typeArgs
| _ -> ()

gives the error:

Unexpected type application in pattern matching. Expected '->' or other token.

But this works:

let typ = typedefof<Dictionary<_,_>>
match dict with
| TypeDef typ typeArgs -> printfn "%A" typeArgs
| _ -> ()

Why is typedefof (or typeof) not allowed here?

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赠佳期 2024-12-02 09:07:23

即使您使用参数化活动模式（其中参数是某个表达式），编译器也会将参数解析为模式（而不是表达式），因此语法受到更多限制。

我认为这本质上与此处讨论的问题相同：如何将复杂表达式传递给参数化活动模式？（我不确定实际的编译器实现，但 F# 规范表示它应该解析为模式）。

作为解决方法，您可以在引用中编写任何表达式，因此您可以这样做：

let undef<'T> : 'T = Unchecked.defaultof<_>

let (|TypeDef|) (typeExpr:Expr) (value:obj) =
  let typeDef = typeExpr.Type.GetGenericTypeDefinition()
  // ...

let dict = System.Collections.Generic.Dictionary<string,obj>()
match dict with
| TypeDef <@ undef<Dictionary<_,_>> @> typeArgs -> printfn "%A" typeArgs
| _ -> ()

Even if you're using a parameterized active pattern (where the argument is some expression), the compiler parses the argument as a pattern (as opposed to an expression), so the syntax is more restricted.

I think this is essentially the same problem as the one discussed here: How can I pass complex expression to parametrized active pattern? (I'm not sure about the actual compiler implementation, but the F# specification says that it should parse as a pattern).

As a workaround, you can write any expression inside a quotation, so you could do this:

let undef<'T> : 'T = Unchecked.defaultof<_>

let (|TypeDef|) (typeExpr:Expr) (value:obj) =
  let typeDef = typeExpr.Type.GetGenericTypeDefinition()
  // ...

let dict = System.Collections.Generic.Dictionary<string,obj>()
match dict with
| TypeDef <@ undef<Dictionary<_,_>> @> typeArgs -> printfn "%A" typeArgs
| _ -> ()

回复收藏 0 原文

氛圍 2024-12-02 09:07:23

添加到托马斯的答案中，这种情况下麻烦的语法似乎与显式类型参数有关。另一种解决方法是使用虚拟参数来传输类型信息

let (|TypeDef|_|) (_:'a) (value:obj) =
  let typeDef = typedefof<'a>
  if obj.ReferenceEquals(value, null) then None
  else
    let typ = value.GetType()
    if typ.IsGenericType && typ.GetGenericTypeDefinition() = typeDef then Some(typ.GetGenericArguments())
    else None

let z = 
    let dict = System.Collections.Generic.Dictionary<string,obj>()
    match dict with
    | TypeDef (null:Dictionary<_,_>) typeArgs -> printfn "%A" typeArgs
    | _ -> ()

Adding to Tomas' answer, the troublesome syntax in this case appears to be with the explicit type arguments. Another workaround is to use a dummy parameter to transmit the type information

let (|TypeDef|_|) (_:'a) (value:obj) =
  let typeDef = typedefof<'a>
  if obj.ReferenceEquals(value, null) then None
  else
    let typ = value.GetType()
    if typ.IsGenericType && typ.GetGenericTypeDefinition() = typeDef then Some(typ.GetGenericArguments())
    else None

let z = 
    let dict = System.Collections.Generic.Dictionary<string,obj>()
    match dict with
    | TypeDef (null:Dictionary<_,_>) typeArgs -> printfn "%A" typeArgs
    | _ -> ()

回复收藏 0 原文

~没有更多了~