Elm Language Program


Example

program is a good pick, when your application does not require any external data for initialization.

It is capable of handling Subscriptions and Commands, which enables way more opportunities for handling I/O, such as HTTP communication or interop with JavaScript.

The initial state is required to return start-up Commands along with the Model.

The initialization of program will require subscriptions to be provided, along with model, view and update.

See the type definition:

program :
    { init : ( model, Cmd msg )
    , update : msg -> model -> ( model, Cmd msg )
    , subscriptions : model -> Sub msg
    , view : model -> Html msg
    }
    -> Program Never

Example

The simplest way to illustrate, how you can use Subscriptions is to setup a simple Port communication with JavaScript.

See how to build the example in Initialize and build / Embedding into HTML

port module Main exposing (..)

import Html exposing (Html, text)
import Html exposing (program)


main : Program Never
main =
    program
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
        }


port input : (Int -> msg) -> Sub msg


-- MODEL


type alias Model =
    Int


init : ( Model, Cmd msg )
init =
    ( 0, Cmd.none )


-- UPDATE


type Msg = Incoming Int


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


-- SUBSCRIPTIONS


subscriptions : Model -> Sub Msg
subscriptions model =
    input Incoming


-- VIEW


view : Model -> Html msg
view model =
    text (toString model)
<!DOCTYPE html>
<html>
    <head>
        <script src='elm.js'></script>
</head>
    <body>
    <div id='app'></div>
    <script>var app = Elm.Main.embed(document.getElementById('app'));</script>
    <button onclick='app.ports.input.send(1);'>send</button>
</body>
</html>