如何在包 Yojson Ocaml 中创建新类型

Question

我有 json 个文件 test.json

{"response":[
    {
        "aid":209228783,
        "thumb_id":"348954492",
        "owner_id":-79421906,
        "title":"title1",
        "description":"description1",
        "created":"1420821911",
        "updated":"1421783832",
        "size":284,
        "can_upload":1
    },
    {
        "aid":205134660,
        "thumb_id":"353880937",
        "owner_id":-79421906,
        "title":"title2",
        "description":"description2",
        "created":"1415386976",
        "updated":"1425057394",
        "size":308,
        "can_upload":0
    },
    {
        "aid":204502901,
        "thumb_id":"347548800",
        "owner_id":-79421906,
        "title":"title3",
        "description":"description3",
        "created":"1414438611",
        "updated":"1419706388",
        "size":1030,
        "can_upload":0
    }
]}

示例，需要从 json 文件中获取值 "aid"、"description" 和 "size"。我创建新类型响应

    type response = {
      aid: int;
      thumb_id: string;
      owner_id: int;
      title: string;
      description: string;
      created: string;
      updated: string;
      size: int;
      can_upload: int
    }

我如何使用它是在我的 json 请求中输入

    let des json =
       [json]
        |> filter_member "response"
        |> flatten
        |> to_list;;
    let json = Yojson.Basic.from_file "test.json" in
    List.iter (fun y -> print_endline(y.aid^"--->"^y.title)) (des json);;

此代码 return 错误类型错误。

Answer 1

（将您的回复记录缩小一点，但仍能说明问题）

type response = {descr:string;
                 size: int;
                 can_upload: int}

let record_list =
  let open Yojson.Basic.Util in 
  let f = Yojson.Basic.from_file "test.json" in
  List.fold_right
    (fun item r_list ->
     {descr = member "description" item |> to_string;
      size = member "size" item |> to_int;
      can_upload = member "can_upload" item |> to_int} :: r_list)
    (member "response" f |> to_list)
    []

Answer 2

Yojson 实际上只是 atdgen 的运行时。将类型定义放入 .atd 文件中，atdgen 将使用 yojson:

为您生成样板代码

(* This is file api.atd *)

type response_item = {
  aid: int;
  thumb_id: string;
  owner_id: int;
  title: string;
  description: string;
  created: string;
  updated: string;
  size: int;
  can_upload: int
}

type response = {
  response: response_item list;
}

Atdgen 生成文件 api_t.mli、api_t.ml、api_j.mli 和 api_j.ml，其中 _t 代表类型，_j 代表JSON。请参阅有关如何将 atdgen 与您的构建系统集成的教程。

然后根据您的 OCaml 代码，您所要做的就是：

let record_list =
  Ag_util.Json.from_file Api_j.read_response "test.json"

如何在包 Yojson Ocaml 中创建新类型

how create new type in package Yojson Ocaml

json

ocaml