f# - 这个 f# echo 服务器出了什么问题？

Question

我写了这个 f# echo 服务器:

open System.Net
open System.Net.Sockets
open System.IO
open System
open System.Text
open System.Collections.Generic

let addr = IPAddress.Parse("127.0.0.1")
let listener = new TcpListener(addr, 2000)
listener.Start()

let rec loop2(c:TcpClient,sr:StreamReader,sw:StreamWriter)=async {
        let line=sr.ReadLine()
        if not(line=null) then
            match line with
                |"quit"->
                    sr.Close()
                    sw.Close()
                    c.Close()  
                 |_ ->
                    if line.Equals("left") then
                        sw.WriteLine("right")
                        return! loop2(c,sr,sw)
                    sw.WriteLine(line)
                    return! loop2(c,sr,sw)

        else
            sr.Close()
            sw.Close()
            c.Close()   
}

let loop()=async {
while true do
    let c=listener.AcceptTcpClient()
    let d = c.GetStream()
    let sr = new StreamReader(d)
    let sw = new StreamWriter(d)
    sw.AutoFlush<-true
    Async.Start(loop2(c,sr,sw))
}

Async.RunSynchronously(loop())

这个程序可以做:

1. 回应客户的信息
2. 当客户说“左”时，返回“右”
3. 当客户端说“退出”时，关闭连接

但是当我运行程序时，当客户端发送“左”、“右”并发送“退出”时，我得到了这个异常:

not handled exception: System.ObjectDisposedException: (con't write to closed) TextWriter。 in Microsoft.FSharp.Control.CancellationTokenOps.Start@1192-1.Invoke(Exception e) in .$Control.loop@419-40(Trampoline this, FSharpFunc2 action) in
Microsoft.FSharp.Control.Trampoline.ExecuteAction(FSharpFunc2 firstAction) in Microsoft.FSharp.Control.TrampolineHolder.Protect(FSharpFunc`2 firstAction) in .$Control.-ctor@476-1.Invoke(Object state) in System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state) in System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, BooleanpreserveSyncCtx) in System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) in System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() in System.Threading.ThreadPoolWorkQueue.Dispatch() in System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() . . .(press any key to continue)

Screenshot of program in action
Screenshot of exception

我该如何解决这个问题？

Answer 1

问题在于，与您在命令式语言中对其同名的期望不同，计算表达式中的 return 不会短路。因此，一旦 if line.Equals("right") 中的 return! 返回，即。套接字关闭后，if block 之后的代码将运行并尝试写入已关闭的套接字。解决方法是将这两行放在 else 中:

                if line.Equals("left") then
                    sw.WriteLine("right")
                    return! loop2(c,sr,sw)
                else
                    sw.WriteLine(line)
                    return! loop2(c,sr,sw)

作为一个额外的样式提示，整个主体可以作为一个匹配来实现:

let rec loop2(c:TcpClient,sr:StreamReader,sw:StreamWriter)=async {
    let line=sr.ReadLine()
    match line with
    | null | "quit" ->
        sr.Close()
        sw.Close()
        c.Close()
    | "left" ->
        sw.WriteLine("right")
        return! loop2(c,sr,sw)
    | _ ->
        sw.WriteLine(line)
        return! loop2(c,sr,sw)
}