异常类型可以通用吗？ [英] Can exception types be generic?

问题描述

我尝试了以下操作，这些操作均无效。

I've tried the following, which don't work.

exception MyError<'a> of 'a
exception 'a MyError of 'a

的MyError是否必须使用long形式：

Do I have to use the long form:

type MyError<'a>(value) =
  inherit System.Exception()
  member this.Value : 'a = value

推荐答案

规范，您必须使用长格式。我没有找到任何解释为什么会这样，但是异常声明的语法看起来像这样（也许也暗示了为什么行为是您所描述的）：

According to the specification, you have to use the long form. I didn't find any explanation why that's the case, but the grammar for exception declarations looks like this (and maybe also hints why the behavior is as you described):

exception-defn：=属性 _opt 异常 union-type-case-data

exception-defn := attributes_opt exception union-type-case-data

union-type-case-data：=

    身份 ;    （空联盟案例）

    标识符 of 类型* ... *类型   （一元联合案例）

     ident：未处理的签名     << i"（i-ary联合案例）

union-type-case-data :=
     ident                                (nullary union case)
     ident of type * ... * type   (n-ary union case)
     ident : uncurried-sig        (n-ary union case)

这很有趣，因为它表明异常声明比类似类型更像区分联合案例。我想您可以想到一个异常声明...

This is quite interesting, because it suggests that exception declarations are more like discriminated union cases than like types. I guess you can think of an exception declaration...

exception MyExn of int

...作为向标准 System.Exception 类型添加新案例的声明这是一个受歧视的工会）。在这种情况下，您将无法使用通用类型参数：

...as a declaration adding new case to the standard System.Exception type (if it was a discriminated union). In this case you wouldn't expect to be able to use a generic type parameter:

type System.Exception = 
  | ...
  | MyExn of int

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