OCaml代码编译报错：类型unit与'a Client.io不匹配

错误的核心是Lwt的绑定操作（>>=）要求回调函数返回Lwt promise类型（即'a Lwt.t，这里显示为'a Client.io），但你所有分支里的printf调用返回的是普通unit类型，和预期的Lwt promise类型不匹配。

你需要让每个分支的返回值都变成unit Lwt.t，有两种简单的实现方式：

方式1：用Lwt.return包装同步打印结果 #

Lwt.return可以把普通值包装成Lwt promise，只需在每个printf后补充Lwt.return ()即可：

if code == 200 then
  if member "usage_type" json <> `Null then
    let usage_type = json |> member "usage_type" |> to_string in
    printf "usage_type: %s\n" usage_type;
    Lwt.return ()
  else
    printf
      "ERROR: The usage_type field requires a paid subscription to the \
       Starter plan or higher.\n";
    Lwt.return ()
else if (code == 400 || code == 401) && member "error" json <> `Null then
  let error_message =
    json |> member "error" |> member "error_message" |> to_string
  in
  print_endline ("ERROR: " ^ error_message);
  Lwt.return ()
else (
  print_endline ("HTTP Code: " ^ Int.to_string code);
  Lwt.return ()
)

方式2：使用Lwt异步打印函数Lwt_io.printf #

Lwt_io.printf是Lwt提供的异步打印函数，直接返回unit Lwt.t，完全符合>>=回调的返回类型要求，代码更简洁：

修改后的完整代码 #

open Printf
open Lwt
open Cohttp
open Cohttp_lwt_unix
open Yojson

let () =
  let ip = "8.8.8.8" in
  let key = "" in
  let uri =
    Uri.of_string
      ("https://api.ip2location.io/?format=json&key=" ^ key ^ "&ip=" ^ ip)
  in
  Lwt_main.run
    ( Client.get uri >>= fun (resp, body) ->
      let code = resp |> Response.status |> Code.code_of_status in
      body |> Cohttp_lwt.Body.to_string >>= fun json_string ->
      let json = Basic.from_string json_string in
      let open Yojson.Basic.Util in
      if code == 200 then
        if member "usage_type" json <> `Null then
          let usage_type = json |> member "usage_type" |> to_string in
          Lwt_io.printf "usage_type: %s\n" usage_type
        else
          Lwt_io.printf
            "ERROR: The usage_type field requires a paid subscription to the \
             Starter plan or higher.\n"
      else if (code == 400 || code == 401) && member "error" json <> `Null then
        let error_message =
          json |> member "error" |> member "error_message" |> to_string
        in
        Lwt_io.printf "ERROR: %s\n" error_message
      else Lwt_io.printf "HTTP Code: %d\n" code )

注意：原代码中printf "ERROR: " ^ error_message是错误写法，因为printf返回unit，无法和字符串做^拼接，上述修改已修复该问题。

内容的提问来源于stack exchange，提问作者Vlam