Question

在 bs-json 库中，提供了一个使用andThen组合器将json结构转换为树的示例。可以在ML接口文件中找到原始示例here on github。将函数decodeTree从此文件复制到浏览器原因工具会显示语法错误。

非常感谢能够让它发挥作用的任何指示。

我尝试将其转换为Reason3会导致类型错误。这是代码：

type tree('a) =
  | Node('a, list(tree('a)))
  | Leaf('a);

let json = {| {
  "type": "node",
  "value": 9
  "children": [{
    "type": "leaf",
    "value": 5,
    "children": [{
      "type": "leaf",
      "value": 3
    }, {
      "type": "leaf",
      "value": 2
    }]
  }, {
      "type": "leaf",
      "value": 4
  }]
} |};

let decodeTree = (decodeValue, json) =>
  Json.Decode.(
    field("type", string)
    |> andThen(type_ =>
         switch type_ {
         | "node" =>
           Node(
             field("value", decodeValue),
             field("children", children =>
               children |> array(decodeTree) |> map(Array.to_list)
             )
           )
         | "leaf" => Leaf(field("value", decodeValue))
         }
       )
  );

let myTree = json |> Json.parseOrRaise |> decodeTree(Json.Decode.int);

这是类型错误，

This has type:
   tree('a)
But somewhere wanted:
   Json.Decode.decoder('b) (defined as (Js.Json.t) => 'b)

Answer 1

很抱歉。不完全确定这个例子发生了什么。这是一个应该有效的方法：

/* Decode a JSON tree structure */
type tree('a) =
  | Node('a, list(tree('a)))
  | Leaf('a);

module Decode = {
  open Json.Decode;

  let rec tree = decoder =>
    field("type", string) |> andThen(
       fun | "node" => node(decoder)
           | "leaf" => leaf(decoder)
           | _      => failwith("unknown node type")
     )

  and node = (decoder, json) =>
    Node(
      json |> field("value", decoder),
      json |> field("children", array(tree(decoder)) |> map(Array.to_list))
    )

  and leaf = (decoder, json) =>
    Leaf(json |> field("value", decoder));
};

let json = {| {
  "type": "node",
  "value": 9,
  "children": [{
    "type": "node",
    "value": 5,
    "children": [{
      "type": "leaf",
      "value": 3
    }, {
      "type": "leaf",
      "value": 2
    }]
  }, {
      "type": "leaf",
      "value": 4
  }]
} |};

let myTree =
  json |> Json.parseOrRaise
       |> Decode.tree(Json.Decode.int);

修改：您遇到的具体错误是andThen期待解码器，但获得tree('a)（Node或Leaf ）。将其转换为解码器所需的只是添加一个json参数，您将其传递给字段解码器：

|> andThen((type_, json) =>
     switch type_ {
     | "node" =>
       Node(
         json |> field("value", decodeValue),
         json |> field("children", children =>
           children |> array(decodeTree) |> map(Array.to_list)
         )
       )
     | "leaf" => json |> Leaf(field("value", decodeValue))
     }
   )

解码json树结构

1 个答案: