在 Elm 中优雅地处理 Flags 中丢失的键

Question

我的应用程序通过标志从本地存储获取初始模型值。我向模型添加了一个新密钥，它在启动 Elm 应用程序时导致错误，因为通过标志传递的值中缺少密钥 ("bar")。考虑到将来可以添加更多新键，而且我不想每次发生时都必须清除 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
        }

HTML代码

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

Answer 1

这是 Elm Slack 频道的 @ilias 友情提供的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>

在 Elm 中优雅地处理 Flags 中丢失的键

Handling missing keys in Flags gracefully in Elm

flags

decoder

elm