Talk (and script) given to Triangle |> Elixir to get acquainted with the Elixir development environment
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(best viewed w/ Deckset, generated slides here)

Play along

Only prerequisite is to have Elixir installed

v1.0 or greater is fine

$ mix -v
Mix 1.0.5

$ elixir -v
Elixir 1.0.5

You are not alone

^ Lots of places to interact, but trying to center local Elixir community in Triangle Devs Slack room.

Getting To Know the Elixir Development Environment

^ When learning a new language, most people focus on syntax and design patterns. Leaves you unprepared for doing actual work (how create project, where do files live etc…)

^ Once have cursory understanding of syntax, I like to understand the language environment and tooling.

^ Hope to show you the lay of the land when it comes to Elixir app dev so you know how to even get to the point to start on your first play app. Meant to be at pace you can follow on your laptop…

What is a language environment?

  • Build tool
  • Dependency resolution
  • Configuration management
  • Testing framework
  • Interactive shell

^ Might seem scattered, but all support the development process

^ These are all table-stakes for modern language - we as devs expect these utilities to be well defined so we can be productive from the start

^ First thing is how to create an app, what dir structure is expected, how app is bootstrapped etc… First tool: Mix


  • Project bootstrapper (like script/rails g)
  • Build utility/script runner (like Rake)
  • Dependency manager (like Bundler)

^ You'll interact a lot with mix because it's the core command line tool of Elixir and has many responsibilities

^ Let's see how it creates projects for us

Create new project*

$ mix help
mix                   # Run the default task (current: mix run)
mix app.start         # Start all registered apps

$ mix new triangle
$ cd triangle
$ ls config    lib       mix.exs   test

^ Look at dir structure: config, lib and test dirs

^ mix.exs: Main entrypoint for app - define sub-apps (Elixir project can have many sub-apps, all composed as a single system), dependencies, metadata

^ Compile w/ mix compile, look at _build for build artifacts. Erlang VM code. Elixir compiles to Erlang bytecode.

^ mix test

^ Tests provide convenient way to get some code executing, so go there next


# test/triangle_test.exs
test "area" do
  assert Triangle.area(3, 5) == 3 * 5 / 2
$ mix test
# lib/triangle.ex
def area(base, height), do: base * height / 2

^ Edit triangle_test.exs. ExUnit is Elixir's unit testing framework

^ area function uses single-line body. Common when simple body definitions

^ Can also run individual tests: mix test test/triangle_test.exs:4

^ Tests are great way to exercise your project, but sometimes need more ad-hoc environment to play in…


  • Interactive elixir, REPL
  • Loads Elixir environment w/ shell access
  • Dynamically reload code

^ IEx is the easiest way to start playing with Elixir and has lot of niceties to help you navigate and get around

IEx playground*

$ iex
> c "lib/triangle.ex"
> Triangle.area 2, 3
> r Triangle
> h Enum.<tab>
$ iex -S mix
> Triangle.area 2, 3

^ Start IEx w/ iex. Do a = 1

^ Do h to show helpers

^ h Enum.<tab> to show tab-completion

^ ls to see FS

^ v 1 to get previous evaluations. Very friendly, navigable environment.

^ c "lib/triangle.ex" to compile and load sample app. Triangle.area 2, 3

^ Edit triangle.ex, and r Triangle to get updated def.

^ Ctrl-C, Ctrl-C to quit

^ Shortcut to load all app dependencies, iex -S mix


  • Print messages to stdout
  • Attach interactive shell to running process (IEx.pry)

Print to stdout*

def area(base, height) do
	IO.puts "base: #{base}, height: #{height}"
	base * height / 2

^ Add puts statement to area and run test

^ IO.puts, IO.inspect

^ Familiar string interpolation available

Attached shell*

# triangle.ex
require IEx
def area(base, height), do: IEx.pry && base * height / 2
$ iex -S mix
> Triangle.area 2, 3
pry(1)> base
pry(2)> respawn

^ IEx.pry attaches to current IEx session, lets you peak/manipulate local scope

^ respawn gets out of IEx.pry, but any changed state is not kept


  • Package manager is called Hex (
  • Dependency resolution handled by Mix
  • Project dependencies defined in mix.exs

^ Three components to dependency management. Publically accessible repo of packages, way to fetch them, and place to define them

Add dependency*

# triangle.ex
require Metrix
def area(base, height) do
  Metrix.measure "triangle.area", fn -> base * height / 2 end
# mix.exs
def application do
  [applications: [:logger, :metrix]]

defp deps do
    {:metrix, "~> 0.2.0"}
$ mix deps.get

^ Add simple metrics library to Triangle as example

^ Edit Triangle, add metrix to application and deps of mix.exs. In applications if it's a sub-app with own supervisor tree. If just a lib, only in deps. Depends on implementation.

^ Notice: it's all code, no YAML

^ Run mix deps.get, look in deps dir to see dependencies locally vendored

^ mix test to see Metrix output


  • config/ contains configurations
  • Config sets up app values

^ Every lang has different way, or no consistent way at all, to define environment-specific configuration. Elixir has first class config support.

^ Look in config/config.exs

Using config values*

# config/config.ex
config :triangle, :default_length, 4
# triangle.ex
def equilateral(length), do: {length, length, length}
def equilateral do
  Application.get_env(:triangle, :default_length))
  |> equilateral
# triangle_test.exs
test "default equilateral side length of 4" do
  assert Triangle.equilateral == {4, 4, 4}

^ Add config value in config/config.exs, first arg must match app atom

^ Add equilateral def to Triangle, test

^ Application.get_env/2 to get values


  • Mix.env available at runtime
  • Environment specific configs in config/

^ Quite often need way to toggle between configs depending on env

^ Uses file-based configurations

Environment configuration*

$ touch config/dev.exs config/test.exs
# config/config.exs
import_config "#{Mix.env}.exs"
# config/dev.exs
use Mix.Config
config :triangle, :default_length, 3
# config/test.exs
use Mix.Config
config :triangle, :default_length, 2

^ Add config/ files for test and dev, add separate length values. Run mix test to see errors

^ Load iex to see dev value: Application.get_env(:triangle, :default_length))

^ Show Mix.env value

^ Actual environment (not app vars) accessible via System.get_env/1

^ I find config/Application.get_env/System.env to be a bit confusing due to the overloading of "environment".


This talk/script can be found at:

I can be found