CheckForUpdates.elm

type Update
    = SmallUpdate
    | BigUpdate
    | EnormousUpdate


validate : Appointment -> Appointment -> List Update
validate originalModel newModel =
    [ differentCustomer
    , differentTime
    , newService
    ]
        |> check originalModel newModel

type alias Validation =
    Appointment -> Appointment -> List Update

check : Appointment -> Appointment -> List (List Validation) -> List Update
check originalModel newModel validations =
    List.map (\v -> v originalModel newModel) validations
        |> List.concat


differentCustomer : Validation
differentCustomer originalModel newModel =
    if originalModel.customer == newModel.customer then
        []
    else
        [ BigUpdate ]

-- etc..


type alias Model =
    { --..
    , updates : List Update
    }


-- Called after every significant update
checkForUpdates : Model -> Model
checkForUpdates model =
    let
        updates =
            validate model.originalAppointment (toAppointment model) 
    in
    if List.isEmpty updates then
        { model 
            | updates = []
            , updateButtonDisabled = True 
        } 
    else
        { model
            | updates = updates
            , updateButtonDisabled = False
        }


update msg model =
    case msg of
        -- ..


        UpdateButtonClicked ->
            model => updatesToCmds model.updates


updatesToCmds : List Update -> Cmd Msg
updatesToCmds =
    List.map fromUpdate >> Cmd.batch


fromUpdate : Update -> Cmd Msg
fromUpdate update =
    case update of
        BigUpdate ->
            Ports.send OneKindOfApiCall

        SmallUpdate ->
            Ports.send AnotherKindOfApiCall