Phoenix and Elm, a real use case (pt. 1)

Introduction to creating a SPA with Phoenix and Elm.
Feb 3, 2017 · 7 min read


It has been a long time since my last post. Last September I joined The Book of Everyone, a very cool startup not only with an awesome product, but with a very passionate and talented team as well, and I have been very busy since then. Apart from working for such a team, another of the main reasons for joining was that most of the current stack is Elixir and Phoenix, so it was an excellent opportunity for solving real problems using some of my favorite technologies. At that time, I also started to play around with Elm, the functional programming language for the web, rewriting some of my pet projects like Phoenix Battleship. As a React/Redux fan and recently converted functional programmer, I felt it was the right path to take. Once I was comfortable enough with the syntax and architecture, I proposed building a proof of concept in Elm as the new front-end for an internal tool that we use at TBOE for creating landing pages. A few weeks later, the POC was ready, pushed to production, and has been working without a single runtime error since then.

I am having so much fun coding in Elm that I can't resist writing a small series about creating a small SPA, covering basic topics such as integrating Elm in a Phoenix project, routing in an Elm program, making HTTP requests and sockets support. So let's get started!

Why Elm?

The people who either know me well or have worked with me know that I am extremely passionate about any new technology that I start learning and I like. However, in the case of Elm, it has been different. I first heard about Elm almost a year ago, while learning Elixir and doing my first pet projects, and I think it was reading some kind of article about Redux. In fact, in thanks to that article, I discovered that Elm's architecture inspired Redux and that caught my attention immediately. After an initial try, I decided that learning Elixir was enough task, and left Elm for later, but I kept reading articles and tutorials about it. It was not until when version 0.17 came out, that I decided to give it another try, and I have been coding Elm almost every day since then, just for the joy of it most of the time. Joy is a word that truly represents to me the experience of coding in Elm because:

  • It is functional and immutable; therefore I do not need to be switching between paradigms while coding back-end and front-end.
  • Its type system in combination with its gentle compiler forces you to write good and error-free code.
  • Talking about the compiler, it has the best error messages I have ever seen. They look more like a tutorial, nothing to do with the error messages we are all so used to. This also makes refactoring really easy and straightforward.
  • It generates good JavaScript free of runtime errors. If it compiles, it works in the browser. True story.
  • It has a decent package manager and repository, which is very strict in terms of publishing packages, and even detects breaking changes in new versions.
  • It is both a language and a framework. If you have tried Redux before, then you will find the Elm Architecture easy to understand and familiar.
  • It has some decent linting and formatting packages for almost any editor, that assist you writing clean and pretty code.
  • Very performant and fast virtual DOM.
  • Once you get it, you experiment a real productivity boost.

And I could probably keep adding more reasons to the list, but let's leave it there and start with the fun part.

The Phoenix application

Before continuing, let's take a closer look at what we are going to build. If you have been following my blog, I am sure you will find it familiar as it is the same concept I have been using since I started writing about Rails and React.

Final result

Starting from a new Phoenix application, let's create the only model we need:

$ mix phoenix.gen.model Contact contacts first_name last_name gender:integer birth_date:date location phone_number email headline:text picture

This creates the following migration file:

# priv/repo/migrations/20160815170103_create_contact.exs

defmodule PhoenixAndElm.Repo.Migrations.CreateContact do
  use Ecto.Migration

  def change do
    create table(:contacts) do
      add :first_name, :string, null: false
      add :last_name, :string, null: false
      add :gender, :integer, default: 0
      add :birth_date, :date, null: false
      add :location, :string, null: false
      add :phone_number, :string
      add :email, :string, null: false
      add :headline, :text
      add :picture, :string


The previous command also creates the basic model/schema with the bunch of fields:

# web/models/contact.ex

defmodule PhoenixAndElm.Contact do
  use PhoenixAndElm.Web, :model

  schema "contacts" do
    field :first_name, :string
    field :last_name, :string
    field :gender, :integer
    field :birth_date, Ecto.Date
    field :location, :string
    field :phone_number, :string
    field :email, :string
    field :headline, :string
    field :picture, :string


  @doc """
  Builds a changeset based on the `struct` and `params`.
  def changeset(struct, params \\ %{}) do
    |> cast(params, [:first_name, :last_name, :gender, :birth_date, :location, :phone_number, :email, :headline, :picture])
    |> validate_required([:first_name, :last_name, :gender, :birth_date, :location, :phone_number, :email, :headline, :picture])

This is all we need at the moment. Eventually, we will be adding full-text search support, but we will see that later on. Don't forget to run the migration task before moving on to the next step:

$ mix ecto.migrate

Installing Elm

In order to add Elm to the project, we first need to install the necessary npm packages:

$ npm install elm elm-brunch --save

We are going to be placing all the Elm stuff in the web/elm folder, so we have to update the brunch-config.js file, adding the following changes:

// brunch-config.js

exports.config = {
  // ...

  paths: {
    watched: [
      // ...

    // ...

  plugins: {
    // ...

    elmBrunch: {
      elmFolder: 'web/elm',
      mainModules: ['Main.elm'],
      outputFolder: '../static/js',

    // ...

  // ...

We are telling brunch to watch the web/elm folder and configuring the elmBrunch plugin. Now that this is set up correctly let's create the web/elm folder and install the first one of the Elm packages that we will be using:

$ mkdir web/elm
$ cd web/elm
$ elm-package install elm-lang/html -y

Doing this not only installs Elm's core and HTML libraries but creates the basic elm-package.json file, which is very similar to npm's package.json file, and the core configuration on any Elm application:

  "version": "1.0.0",
  "summary": "helpful summary of your project, less than 80 characters",
  "repository": "",
  "license": "BSD3",
  "source-directories": [
  "exposed-modules": [],
  "dependencies": {
    "elm-lang/core": "5.0.0 <= v < 6.0.0",
    "elm-lang/html": "2.0.0 <= v < 3.0.0",
  "elm-version": "0.18.0 <= v < 0.19.0"

Now we can add a very basic Main.elm file with the Main module that simply returns a Hello, World! message:

--- web/elm/Main.elm

module Main exposing (..)

import Html exposing (Html, text)

main : Html a
main =
    text "Hello, World!"

Next, we need to update the main app.js file to import the javascript generated by Elm and render the result:

// web/static/js/app.js

import Elm from './main';

const elmDiv = document.querySelector('#elm_target');

if (elmDiv) {

We want to embed it in a div with the elm_target id, so let's go ahead and add that div in the corresponding Phoenix template:

<!-- web/templates/page/index.html.eex -->

<div id="elm_target"></div>

Now we are ready to start the Phoenix server and check out that everything is working fine. The output should be something similar to this:

$ iex -S mix phoenix.server
Erlang/OTP 19 [erts-8.1] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
info] Running PhoenixAndElm.Endpoint with Cowboy using http://localhost:4000
Interactive Elixir (1.4.0) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Elm compile: Main.elm, in web/elm, to ../static/js/main.js
[BABEL] Note: The code generator has deoptimised the styling of "web/static/js/main.js" as it exceeds the max of "100KB".
10:27:47 - info: compiling
10:27:50 - info: compiled 89 files into 2 files, copied 2 in 15.2 sec

Visiting http://localhost:4000 should render the Hello, World! message in the browser.

Enough for now. In the next part, we will dig a bit more in the Elm architecture, define our application's state and start coding our first Elm modules. In the meantime, you can take a look to the repository with the final result or see it in action in the live demo.

Happy coding!