Running the project

We typically run Gleam projects in 4 ways:


Gleam tests are written using the Erlang test framework eunit.

To write a test add a function to a module in the test, giving the function a name that ends in _test. The gleam/should module can be used to make assertions about the behaviour of our code.

import gleam/should
import my_fantastic_library

pub fn addition_test() {
  |> should.equal("Hello from my_fantastic_library!")

Once written the tests can be run using the rebar3 eunit command.

The shell

An interactive Erlang shell can be started using the rebar3 build tool:

rebar3 shell
# ===> Verifying dependencies...
# ===> Compiling gleam_stdlib
# Erlang/OTP 22 [erts-10.4.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1]
# Eshell V10.4.3  (abort with ^G)
# 1>

Here we can try out our functions by typing them in:

1> my_fantastic_library:hello_world().
# <<"Hello from my_fantastic_library">>

It’s important to remember that this is an Erlang shell rather than a Gleam shell, so Erlang syntax must be used. Don’t forget to put a . at the end of the expression otherwise the shell won’t do anything.


To be run in production Erlang based applications are build into a deployable bundle called a release.

At a later date we will have built in support and documentation for release, but for now please refer to these Erlang docs:

Using Escripts

An escript is an option for making command line Gleam programs. Using Erlang’s escriptize generates an escript executable containing the project’s and its dependencies’ BEAM files.

Running escriptize creates an executable file:

_build/default/bin/my_project_name which requires a fn main as the entrypoint

An example main function signature would look like this:

import gleam/list

pub external type CharList

external fn char_list_to_string(CharList) -> String =
  "erlang" "list_to_binary"

pub fn main(args: List(CharList)) {
  let args =, char_list_to_string)
  todo("Your code goes here")
# Build the project
rebar3 compile

# Run to enable escriptize IO commandline tooling
rebar3 escriptize

# Run the program!