F# 编译器警告 FS0067 的 when 守卫异常

Question

在F#中，我们可以方便地使用failwith和 failwithf , 两者都抛出 Exception 实例。因此，您可能偶尔需要使用“何时保护”来区分不同的异常条件。

这是一个说明性示例:

    try
        let isWednesday = DateTime.Now.DayOfWeek = DayOfWeek.Wednesday
        if isWednesday then failwith "Do not run this on Wednesdays!"
    with
    | :? DivideByZeroException -> printfn "You divided by zero."
    | :? Exception as ex when ex.Message.Contains("Wednesdays") -> printfn "I said, %s" ex.Message
    | ex -> printfn "%s" ex.Message

但是，上面的代码会导致两个警告:

Program.fs(14,7): warning FS0067: This type test or downcast will always hold 
Program.fs(14,7): warning FS0067: This type test or downcast will always hold 

我如何避免该警告？

Answer 1

删除 Exception 的类型测试模式类(class)。这是不必要的。

with
| :? DivideByZeroException -> printfn "You divided by zero."
| ex when ex.Message.Contains("Wednesdays") -> printfn "I said, %s" ex.Message
| ex -> printfn "%s" ex.Message