# `pete-murphy/elm-loadable` A data type for tracking loadable (and reloadable) data, like data fetched from a backend server. ## Installation ``` elm install pete-murphy/elm-loadable ``` ## Design Goals This package is centered around the `Loadable` type, which is a minimal extension of `RemoteData` from [`krisajenkins/remotedata`](https://package.elm-lang.org/packages/krisajenkins/remotedata/latest) that adds a "reloading" state to the error and success cases. See [this discussion on alternative solutions to this problem](https://github.com/krisajenkins/remotedata/issues/9) for background.

Differences from RemoteData

`RemoteData e a` is a sum type defined as ```elm type RemoteData e a = NotAsked | Loading | Failure e | Success a ``` As the "sum type" name implies, we can think of this type as having $$2 + e + a$$ inhabitants (see [_The algebra (and calculus!) of algebraic data types_](https://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types)). Under this lens, `Loadable e a` is a _product of sums_ represented internally as ```elm type Value e a = Empty | Failure e | Success a ``` with a `loading : Boolean` added to all cases. Expressed algebraically, `Value` is $$1 + e + a$$ (very similar to `RemoteData`) and `Loadable` is the product of that sum _and_ `Bool`'s 2 inhabitants: $$2 \times (1 + e + a)$$ or $$2 + 2e + 2a$$ (compare to `RemoteData`'s $$2 + e + a$$). You can losslessly convert from `RemoteData` to `Loadable` ```elm remoteDataToLoadable : RemoteData e a -> Loadable e a remoteDataToLoadable remoteData = case remoteData of RemoteData.NotAsked -> Loadable.notAsked RemoteData.Loading -> Loadable.loading RemoteData.Success a -> Loadable.succeed a RemoteData.Failure e -> Loadable.fail e ``` Going the other direction just means discarding the loading state in the success & error cases. ```elm loadableToRemoteData : Loadable e a -> RemoteData e a loadableToRemoteData loadable = case ( Loadable.value loadable, Loadable.isLoading loadable ) of ( Loadable.Empty, False ) -> RemoteData.NotAsked ( Loadable.Empty, True ) -> RemoteData.Loading ( Loadable.Success a, _ ) -> RemoteData.Success a ( Loadable.Failure e, _ ) -> RemoteData.Failure e ```

## Overview ### Example usage ```elm type alias Model = { todos : Loadable Http.Error (List Todo) } init : ( Model, Cmd Msg ) init = ({ todos = Loadable.notAsked }, fetchTodos ) type Msg = UserClickedFetchTodos | BackendRespondedWithTodos (Result Http.Error (List Todo)) update : Msg -> Model -> ( Model, Cmd Msg ) update msg model = case msg of UserClickedFetchTodos -> ( { model | todos = Loadable.toLoading model.todos }, fetchTodos ) BackendRespondedWithTodos result -> ( { model | todos = Loadable.fromResult result }, Cmd.none ) view : Model -> Html Msg view model = Html.div [] [ Html.h1 [] [ Html.text "Todos" ] , Html.button [ Events.onClick UserClickedFetchTodos ] [ Html.text "Fetch Todos" ] , Html.div [ Attributes.classList [ ( "loading", Loadable.isLoading model.todos ) ] ] [ case Loadable.value model.todos of Loadable.Empty -> Html.text "Loading..." Loadable.Success todos -> Html.ul [] (List.map viewTodo todos) Loadable.Failure _ -> Html.text "Something went wrong" ] ] ```