列表中的 Json.Decode.Pipeline (elm 0.18)

Question

我有一个带有嵌套列表的类型别名，我想用 Json.Decode.Pipeline 解析它.

import Json.Decode as Decode exposing (..)
import Json.Encode as Encode exposing (..)
import Json.Decode.Pipeline as Pipeline exposing (decode, required)

type alias Student =
    { name : String
    , age : Int
    }

type alias CollegeClass =
    { courseId : Int
    , title : String
    , teacher : String
    , students : List Student
    }

collegeClassDecoder : Decoder CollegeClass
collegeClassDecoder =
    decode CollegeClass
        |> Pipeline.required "courseId" Decode.int
        |> Pipeline.required "title" Decode.string
        |> Pipeline.required "teacher" Decode.string
        |> -- what goes here?

这是如何运作的？

Answer 1

您需要将解码器传递给 Decode.list .在您的情况下，它将是根据您的 Student 的形状定制的。类型。

这尚未经过测试，但类似以下内容应该可以工作:

studentDecoder =
    decode Student
        |> required "name" Decode.string
        |> required "age" Decode.int

collegeClassDecoder : Decoder CollegeClass
collegeClassDecoder =
    decode CollegeClass
        |> Pipeline.required "courseId" Decode.int
        |> Pipeline.required "title" Decode.string
        |> Pipeline.required "teacher" Decode.string
        |> Pipeline.required "students" (Decode.list studentDecoder)

见 this关于编写自定义标志解码器的帖子，这应该是有指导意义的。