This project illustrates how to embed an Elm program in an HTML page and how to communicate with JavaScript.

git clone https://github.com/evancz/elm-html-and-js.git
cd elm-html-and-js
elm-make Stamps.elm --output elm.js
open index.html

If you create an Elm module named Stamps, it will be named Elm.Stamps in JavaScript. To instantiate a module you can use any of the following functions:

Elm.fullscreen(Elm.Stamps, {reset:[]});   // take over the whole page
Elm.embed(Elm.Stamps, div, {reset:[]});   // embed in a specific DOM node
Elm.worker(Elm.Stamps,     {reset:[]});   // instantiate without graphics

Each of these creates a module instance that you can communicate with from JavaScript.

// Embed the Stamps module in a div with ID 'elm-stamps'
var div = document.getElementById('elm-stamps');
var stamps = Elm.embed(Elm.Stamps, div, {reset:[]});

// You can send and receive values through
// ports 'reset' and 'count'.
stamps.ports.reset.send([]);
stamps.ports.count.subscribe(function(event) {
    console.log(event.value);
});

Communication between Elm and JavaScript happens by sending events through ports, like reset and count. Elm interprets these ports as signals and JavaScript interprets them as event streams. In Elm, you declare ports like this:

-- incoming reset events
port reset : Signal ()

-- outgoing count of stamps
port count : Signal Int
port count = length <~ stamps

Values passed along named channels must conform to these rules.