f# - 重试 monad 和零构造

Question

我正在尝试使用从我们心爱的堆栈溢出中获取的 Retry Monad:

type RetryBuilder(max, sleep : TimeSpan) = 
      member x.Return(a) = a
      member x.Delay(f) = f
      member x.Zero() = failwith "Zero"
      member x.Run(f) =
        let rec loop(n) = 
            if n = 0 then failwith "Failed"
            else 
                try f() 
                with ex -> 
                    sprintf "Call failed with %s. Retrying." ex.Message |> printfn "%s"
                    Thread.Sleep(sleep); 
                    loop(n-1)
        loop max

我想用它来使我的文件复制代码更加健壮:

let retry = RetryBuilder(3, TimeSpan.FromSeconds(1.))
retry {
    System.IO.File.Move("a", "b")
}

现在我注意到它有时会因“零”异常而失败。我试图删除 member x.Zero() = failwith "Zero"但现在我收到一个编译时错误:

This construct can be only used if builder defines a 'Zero' method.

任何想法如何继续？

Answer 1

Lee 建议您可以使用 return ()在否则会抛出的计算结束时，因为它们调用 Zero成员。这是一个很好的技巧 - 但您实际上可以将其直接集成到计算构建器中。
Zero当您的计算结束而不返回时使用成员。您可以将其更改为与 return () 相同的操作:

type RetryBuilder(max, sleep : TimeSpan) = 
  member x.Return(a) = ...
  member x.Zero() = x.Return( () )

然后你可以只写原始代码，你会得到单元结果:

let retry = RetryBuilder(3, TimeSpan.FromSeconds(1.))
retry {
  System.IO.File.Move("a", "b")
}