OCaml:预期Lwt表达的类型单位为Lwt.t [英] OCaml: Lwt expression was expected of type unit Lwt.t

问题描述

我想为我的OUnit测试写一个小的服务器，当客户端连接到套接字时，将数据发送到客户端并退出.

I wanted to write for my OUnit tests a little server that when a client connect on a socket, send data to the client and quit.

因为我想在测试中使用它，所以我认为应该在线程中创建服务器，并在线程中创建客户端.所以这是代码:

Because I wanted to use this in my tests, I thought that I should create a server in a thread and create a client in a thread. So here is the code:

open Sys
open Unix
open Lwt

(* a simple server that send a message when a client  connects
 * compile with
 * ocamlfind ocamlc -o lwt_server -package lwt,lwt.unix,unix  -linkpkg -g lwt_server.ml
 * http://baturin.org/code/lwt-counter-server/
 * https://mirage.io/wiki/tutorial-lwt
 * http://aubedesheros.blogspot.fr/2011/05/ocaml-lwt-mini-tutorial.html
 *)

let host = Unix.inet_addr_loopback (* 127.0.0.1 *)
let port = 6600
let max_pending_request = 10

(* Initialize logging capabilities at INFO level *)

let () = Lwt_log.add_rule "*" Lwt_log.Info

let send_message oc =
  Lwt_io.write_line oc "test"

let accept_connection conn =
  let fd, _ = conn in
  let oc = Lwt_io.of_fd Lwt_io.Output fd in
  Lwt.on_failure (send_message oc ) (fun e -> Lwt_log.ign_error (Printexc.to_string e));
  Lwt_log.info "New connection" >>= return

let create_server sock =
  let serve () =
    Lwt_unix.accept sock >>= accept_connection
  in serve

let sock_recv sock maxlen =
  let str = Bytes.create maxlen in
  let recvlen  = recv sock str 0 maxlen [] in
  String.sub str 0 recvlen

let sock_read sock =
  let answer = sock_recv sock 512 in
    Lwt_io.write_line Lwt_io.stdout answer

let create_socket () =
  let sock = Lwt_unix.socket PF_INET SOCK_STREAM 0 in
  Lwt_unix.bind sock @@ ADDR_INET(host, port);
  Lwt_unix.listen sock max_pending_request;
  sock


let () =
  let sock = create_socket () in
    let threads = Lwt.join [create_server sock; sock_read sock] in
      Lwt_main.run threads

这是我在oder中用来编译它的命令和错误消息

Here is the command I use in oder to compile it and the error message

ocamlfind ocamlc -o lwt_server -package lwt,lwt.unix,unix  -linkpkg -g lwt_server.ml
File "lwt_server.ml", line 53, characters 28-46:
Error: This expression has type unit -> unit Lwt.t
       but an expression was expected of type unit Lwt.t

第53行是指(不开玩笑，我已经检查了两次):

The line 53 refers to (no joke, I have checked twice):

        let threads = Lwt.join [create_server sock; sock_read sock] in

这是我第一次使用Lwt，我不确定这是否是我应该与OUnit一起使用的解决方案.

It is the first time I use Lwt, I am not sure if it is the solution I should use with OUnit.

推荐答案

问题在于，create_server sock返回类型为unit -> unit Lwt.t的另一个函数，而不是类型错误所指示的Lwt线程(unit Lwt.t):

The issue is that create_server sock returns another function of type unit -> unit Lwt.t rather than an Lwt thread (unit Lwt.t) as indicated in the type error:

let create_server sock =
  let serve () =
    Lwt_unix.accept sock >>= accept_connection
  in serve (* serve is a function, not a thread *)

您可以重写此代码以返回线程，而不是用于修复此类型错误的函数:

You can rewrite this to return a thread rather than a function to fix your this type error:

let create_server sock =
  Lwt_unix.accept sock >>= accept_connection

我还没有对代码进行整体测试，但这应该可以解决类型错误.

I haven't tested the code as a whole, but this should fix the type error.

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