Elm：将JSON解码为简单的记录结构

Question

我正在努力寻找正确的方法来完成我的解码器。我从以下格式的数据开始：

[{_id:'interests', [{obj1}, {obj1}]}
,{_id:'countries', [{obj2}, {...}]} 
,{_id:'sections', [{obj3}, {...}]}]

我想要到达一个解码器摘要。

type alias Summary =
    { sections : List Section
    , interests : List Interest
    , countries : List Country
    }

到目前为止，我能够得到的最好输出是这样的：

[ Interests (List Interest), Countries (List Country), Sections (List Section)]

但仍需要一些脆弱的模式匹配（依赖于数组的一致顺序，因此在 0.16 版本中存在很大问题）。为此，我使用

summaryItemDecoder : String -> Decoder SummaryInfo
summaryItemDecoder item =
    let dec =
        case item of
            "sections" -> Json.map Sections sectionsDecoder
            "interests" -> Json.map Interests interestsDecoder
            "countries" -> Json.map Countries countriesDecoder
    in ("data" := dec)

listSummaryDecoder : Decoder (List SummaryInfo)
listSummaryDecoder =
    ("_id" := string) `Json.andThen` summaryItemDecoder
    |> list

完整代码在这里。感激一些最后的提示。

- Simon H

Answer 1

我不确定你是否能做得更好；你试图解析一种可以表达你的类型无法表示的东西的格式，所以你唯一的选择就是失败。

为了满足模式匹配的需求，也许你可以在你的fail解码器中加入一个otherwise子句？(http://package.elm-lang.org/packages/elm-lang/core/3.0.0/Json-Decode#fail)

- grumpyjames

---

谢谢，我确实添加了一个全覆盖模式，但是真的希望能找到更灵活的输出。 - Simon H

Answer 2

您可以将信息列表折叠成摘要。

type Interest = Interest
type Section = Section
type Country = Contry

type Info =  Interests (List Interest) | Countries (List Country) | Sections (List Section) 


type alias Summary =
    { sections : List Section
    , interests : List Interest
    , countries : List Country
    }

emptySummary : Summary
emptySummary = 
    { sections = []
    , interests = []
    , countries = []
    }


toSummary : List Info -> Summary
toSummary xs = 
  let 
    insert info sum =
      case info of 
        Sections ss -> { sum | sections = ss }
        Interests is -> { sum | interests = is }
        Countries cs -> { sum | countries = cs } 

  in
    List.foldl insert emptySummary xs

infoList : List Info
infoList = []

summary = toSummary infoList 

- pdamoc