Web Apps

So far we have been creating Elm programs with Browser.element, allowing us to take over a single node in a larger application. This is great for introducing Elm at work (as described here) but what happens after that? How can we use Elm more extensively?

In this chapter, we will learn how to create a “web app” with a bunch of different pages that all integrate nicely with each other, but we must start by controlling a single page.

Control the Document

The first step is to switch to starting programs with Browser.document:

document :
  { init : flags -> ( model, Cmd msg )
  , view : model -> Document msg
  , update : msg -> model -> ( model, Cmd msg )
  , subscriptions : model -> Sub msg
  -> Program flags model msg

The arguments are almost exactly the same as Browser.element, except for the view function. Rather than returning an Html value, you return a Document like this:

type alias Document msg =
  { title : String
  , body : List (Html msg)

This gives you control over the <title> and the <body> of the document. Perhaps your program downloads some data and that helps you determine a more specific title. Now you can just change it in your view function!

Serve the Page

The compiler produces HTML by default, so you can compile your code like this:

elm make src/Main.elm

The output will be a file named index.html that you can serve like any other HTML file. That works fine, but you can get a bit more flexibility by (1) compiling Elm to JavaScript and (2) making your own custom HTML file. To take that path, you compile like this:

elm make src/Main.elm --output=main.js

This will produce main.js which you can load from a custom HTML file like this:

  <meta charset="UTF-8">
  <link rel="stylesheet" href="whatever-you-want.css">
  <script src="main.js"></script>
  <script>var app = Elm.Main.init();</script>

This HTML is pretty simple. You load whatever you need in the <head> and you initialize your Elm program in the <body>. The Elm program will take it from there and render everything.

Either way, now you have some HTML that you can send to browsers. You can get that HTML to people with free services like GitHub Pages or Netlify, or maybe you make your own server and run a VPS with a service like Digital Ocean. Whatever works for you! You just need a way to get HTML into a browser.

Note 1: Creating custom HTML is helpful if you are doing something custom with CSS. Many people use projects like rtfeldman/elm-css to handle all of their styles from within Elm, but maybe you are working in a team where there is lots of predefined CSS. Maybe the team is even using one of those CSS preprocessors. That is all fine. Just load the final CSS file in the <head> of your HTML file.

Note 2: The Digital Ocean link above is a referral link, so if you sign up through that and end up using the service, we get a $25 credit towards our hosting costs for elm-lang.org and package.elm-lang.org.

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