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在Elm中优雅地处理标志中丢失的键

decoder flags elm

Jason O. · 技术社区 · 7 年前

我的应用程序通过标志从localstorage获取初始模型值。我向模型中添加了一个新键,它在启动Elm应用程序时会导致错误,因为通过标志传递的值中缺少键(“bar”)。考虑到将来可以添加更多的新键,并且我不想每次都必须清除localstorage,有没有办法告诉Elm在标志中缺少键时分配默认值?

type alias Model =
    { foo : String, bar : Int }

update : msg -> Model -> ( Model, Cmd msg )
update _ model =
    model ! []

view : Model -> Html msg
view model =
    text <| toString model

main : Program Flags Model msg
main =
    Html.programWithFlags
        { init = init
        , update = update
        , view = view
        , subscriptions = always Sub.none
        }

<body>
  <script>
    var app = Elm.Main.fullscreen({foo: "abc"})
  </script>
</body>

1 回复 | 直到 7 年前

Jason O. 7 年前

Elm Slack channel的@ilias提供了一个很好的解决方案。

https://ellie-app.com/mWrNyQWYBa1/0

module Main exposing (main)

import Html exposing (Html, text)
import Json.Decode as Decode exposing (Decoder)
import Json.Decode.Extra as Decode  --"elm-community/json-extra"


type alias Model =
    { foo : String, bar : Int }


flagsDecoder : Decoder Model
flagsDecoder =
    Decode.map2 Model
        (Decode.field "foo" Decode.string |> Decode.withDefault "hello")
        (Decode.field "bar" Decode.int |> Decode.withDefault 12)


init : Decode.Value -> ( Model, Cmd msg )
init flags =
    case Decode.decodeValue flagsDecoder flags of
        Err _ ->
            Debug.crash "gracefully handle complete failure"

        Ok model ->
            ( model, Cmd.none )


update : msg -> Model -> ( Model, Cmd msg )
update _ model =
    model ! []


view : Model -> Html msg
view model =
    text <| toString model


main : Program Decode.Value Model msg
main =
    Html.programWithFlags
        { init = init
        , update = update
        , view = view
        , subscriptions = always Sub.none
        }

HTML

<body>
  <script>
    var app = Elm.Main.fullscreen({foo: "abc"})
  </script>
</body>