There’s a really cool new BEAM language out there called Gleam. It’s super early days for the language, but what it offers (strong type safety) I think is worth some experimentation for folks that are interested. Lucky enough for those of us currently using Elixir it’s trivially easy to use Gleam code in your Elixir applications. Today I’m going to show you how.

Set up your umbrealla

Gleam compiles to Erlang and uses rebar3 as its build tool, and luckily Mix knows how to work with rebar3 just fine. So what we’re going to do is to set up an umbrella application, where one application is a standard Mix app, and the other one is a standard Gleam app. Start off with this to generate a new Phoenix application that sets up an umbrella app for you.

mix phx . new -- umbrella gleam_example

Now that we have our two applications - :gleam_example and :gleam_example_web , we are going to delete that apps/gleam_example directory and set it up from scratch again as a Gleam application.

Getting Gleam working

First, you’ll need to have Gleam and rebar3 installed, which you can easily do with asdf . Then, to start a new Gleam application we have a familiar CLI to work with:

gleam new gleam_example

This generates the standard structure of a new gleam application. You should now be able to cd into that directory and run the tests with rebar3 eunit . Once that’s all working, you’ve got the basics all hooked up alredy! Yay!

Connecting the dots

Now that we have our two applications, we need to update a few things. First off, we need to tell Mix that our Gleam project needs to use rebar3 for compilation, so we can go to apps/gleam_example_web/mix.exs and we’ll go down to the deps function. We’re going to look for the declaration of the dependency that looks like this {:gleam_example, in_umbrella: true} and we’re going to add one more configuration option at the end that gets this all set for us: {:gleam_example, in_umbrella: true, manager: :rebar3} .

Now if we try and start our server, it’ll work, but we’ll still see a warning in our terminal that looks like this:

warning : path "apps/gleam_example" is a directory but it has no mix . exs . Mix won ' t consider this directory as part of your umbrella application . Please add a "mix.exs" or set the ":apps" key in your umbrella configuration with all relevant apps names as atoms

I don’t like warnings, so let’s fix this. What’s happening here is Mix is saying “I found this directory in your /apps folder, but it doesn’t look like it’s a Mix project, so why are you telling me to compile it?!” So let’s tell Mix specifically what apps we want it to compile for us by going to the top level mix.exs file. There we’ll see configuration that looks like this:

def project do [ apps_path : "apps" , start_permanent : Mix . env ( ) == :prod , deps : deps ( ) ] end

Because we’ve set apps_path: "apps" , Mix assumes that all directories there are Mix applications. We need to tell Mix that this is not true by explicitly adding an apps: key to that configuration:

def project do [ apps_path : "apps" , apps : [ :gleam_example_web ] , start_permanent : Mix . env ( ) == :prod , deps : deps ( ) ] end

Now it will only try and compile the Elixir application for us and not the Gleam application!

The results

Now that we can start our server with mix phx.server and we don’t get any warnings, let’s actually show something that’s called in Gleam. We can go to our PageController that’s automatically generated for us and make the cannonical Hello, world! example. Luckily, because Gleam compiles to Erlang, it looks pretty familiar:

defmodule GleamExampleWeb.PageController do use GleamExampleWeb , :controller def index ( conn , _params ) do text ( conn , :gleam_example . hello_world ( ) ) end end

And that’s it! Now we have a Phoenix application that calls Gleam code!

The only thing that doesn’t work super well right off the bat is running tests for your entire umbrella app, and that’s because Mix doesn’t know how to run tests for rebar3 projects when you run mix test . I kind of get that, and I’m ok with it for now. If you want to have a single command that runs all your tests, you can put something like this in a bash script or a Makefile: mix test && (cd apps/gleam_example && rebar3 eunit) .

If you want to check out the final results for this little example project, it’s on GitHub.