JSON decoding over ports in Elm.

App.elm

port module App exposing (..)

import Html exposing (..)
import Json.Decode.Pipeline exposing (..)
import Json.Decode exposing (..)


port workerUpdated : (Json.Decode.Value -> msg) -> Sub msg


type alias Model =
    { items : List Worker }


type alias Worker =
    { workerActivityName : String
    , sid : String
    , workerName : String
    , workerActivityName : String
    , department : List String
    }


decodeWorkers : Json.Decode.Decoder Model
decodeWorkers =
    Json.Decode.Pipeline.decode Model
        |> Json.Decode.Pipeline.required "items" (Json.Decode.list decodeWorker)


decodeWorker : Json.Decode.Decoder Worker
decodeWorker =
    Json.Decode.Pipeline.decode Worker
        |> Json.Decode.Pipeline.requiredAt [ "value", "WorkerActivityName" ] (Json.Decode.string)
        |> Json.Decode.Pipeline.requiredAt [ "value", "Sid" ] (Json.Decode.string)
        |> Json.Decode.Pipeline.requiredAt [ "value", "WorkerName" ] (Json.Decode.string)
        |> Json.Decode.Pipeline.requiredAt [ "value", "WorkerActivityName" ] (Json.Decode.string)
        |> Json.Decode.Pipeline.custom
            (Json.Decode.at [ "value", "WorkerAttributes", "department" ] (Json.Decode.list Json.Decode.string))


init : ( Model, Cmd Msg )
init =
    ( { items = [] }, Cmd.none )


type Msg
    = NoOp
    | SetModel Model


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        SetModel newModel ->
            ( newModel
            , Cmd.none
            )

        NoOp ->
            ( model
            , Cmd.none
            )


view : Model -> Html Msg
view model =
    div []
        [ viewWorkers model.items
        ]


viewWorkers : List Worker -> Html Msg
viewWorkers items =
    items
        |> List.sortBy .department
        |> List.map viewItem
        |> ul []


viewItem : Worker -> Html Msg
viewItem item =
    li [] [ text (item.workerName ++ " - " ++ item.workerActivityName) ]


mapWorkerUpdated : Json.Decode.Value -> Msg
mapWorkerUpdated modelJson =
    case (decodeModel modelJson) of
        Ok model ->
            SetModel model

        Err errorMessage ->
            let
                _ =
                    Debug.log "Error in mapWorkerUpdated:" errorMessage
            in
                NoOp


decodeModel : Json.Decode.Value -> Result String Model
decodeModel modelJson =
    Json.Decode.decodeValue decodeWorkers modelJson


subscriptions : Model -> Sub Msg
subscriptions model =
    workerUpdated mapWorkerUpdated

example.json

{  
   "items":[  
      {  
         "key":"uiory7943hiufnljddf",
         "value":{  
            "WorkerActivityName":"Foo",
            "EventType":"worker.activity.update",
            "ResourceType":"worker",
            "WorkerPreviousActivitySid":"djkhf98384UJKDF",
            "WorkerTimeInPreviousActivity":"922",
            "Timestamp":"1480346909",
            "WorkerActivitySid":"WJKDUDF89D7FYDFYG",
            "AccountSid":"T8403UJKDFNKDBDFNMDFMN",
            "WorkerName":"Elm Noob",
            "Sid":"DFBJDF8334343",
            "WorkerSid":"DFJIODFND38YUH3IR",
            "WorkspaceSid":"TUIPJDFNNBMFNMFJHK",
            "WorkspaceName":"Josh Hornby",
            "EventDescription":"Trying Elm!",
            "ResourceSid":"9034ndfdfdfdf",
            "WorkerAttributes":{  
               "department":[  
                  "foo"
               ]
            }
         }
      }
   ]
}