Building and Sharing Your First Elixir Library, by Steve Grossi

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![Indy Elixir](https://www.indyelixir.org/images/indyelixir.svg)

# Building and Sharing Your First Elixir Library

### Steve Grossi @ Lessonly

---

# What We’ll Learn

- Mix
- Hex
- Elixir library structure
- Example TDD flow
- Writing and generating documentation
- Doctests
- Publishing our library and docs

---

# What is Hex?

- Package manager for the Erlang ecosystem
- Online repository: [hex.pm](https://hex.pm)

---

# What is Mix?

- Build tool (compile, test, run code)
- Dependency manager
- Environment manager

We use Mix to create Elixir libraries and Hex to package and share them

---

# Creating a new Elixir project

```shell
$ mix new fibonacci
* creating README.md
* creating .gitignore
* creating mix.exs
* creating config
* creating config/config.exs
* creating lib
* creating lib/fibonacci.ex
* creating test
* creating test/test_helper.exs
* creating test/fibonacci_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

cd fibonacci
    mix test

Run "mix help" for more commands.
```

---

# Elixir Library File Structure

```shell
$ tree fibonacci
├── README.md
├── config
│   └── config.exs
├── lib
│   └── fibonacci.ex
├── mix.exs
└── test
    ├── fibonacci_test.exs
    └── test_helper.exs
```

---

# Project Metadata

```elixir
# mix.exs
defmodule Fibonacci.Mixfile do
  use Mix.Project

def project do
    [app: :fibonacci,
     version: "0.1.0",
     elixir: "~> 1.3",
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     deps: deps()]
  end
```

---

# OTP Applications

```elixir
# mix.exs
defmodule Fibonacci.Mixfile do
  # ...

# Configuration for the OTP application
  #
  # Type "mix help compile.app" for more information
  def application do
    [applications: [:logger]]
  end
```

---

# Dependencies

```elixir
defmodule Fibonacci.Mixfile do
  # ...

# Dependencies can be Hex packages:
  #
  #   {:mydep, "~> 0.3.0"}
  #
  # Or git/path repositories:
  #
  #   {:mydep, git: "https://github.com/elixir-lang/mydep.git", tag: "0.1.0"}
  #
  # Type "mix help deps" for more examples and options
  defp deps do
    []
  end
```

---

# Test-Driven Development

```shell
$ mix test
.

Finished in 0.02 seconds
1 test, 0 failures

Randomized with seed 241771
```

---

# Oh, hello there!

```elixir
defmodule FibonacciTest do
  use ExUnit.Case
  doctest Fibonacci

test "the truth" do
    assert 1 + 1 == 2
  end
end
```

---

# Oh, hello there!

```elixir
defmodule FibonacciTest do
  use ExUnit.Case
  doctest Fibonacci

test "the truth" do
    assert 1 + 1 == :nope
  end
end
```

---

# Nope!

```shell
$ mix test

1) test the truth (FibonacciTest)
     test/fibonacci_test.exs:5
     Assertion with == failed
     code: 1 + 1 == :nope
     lhs:  2
     rhs:  :nope
     stacktrace:
       test/fibonacci_test.exs:6: (test)

Finished in 0.03 seconds
1 test, 1 failure
```

---

# Test-Driven Development

```elixir
defmodule FibonacciTest do
  use ExUnit.Case
  doctest Fibonacci

test "returns list of specified length" do
    assert Fibonacci.series(0) == []
    assert Fibonacci.series(1) == [0]
    assert Fibonacci.series(8) == [0, 1, 1, 2, 3, 5, 8, 13]
  end
end
```

---

# Test-Driven Development

```shell
$ mix test

1) test returns list of specified length (FibonacciTest)
     test/fibonacci_test.exs:5
     ** (UndefinedFunctionError) function Fibonacci.series/1 is undefined or private
     stacktrace:
       (fibonacci) Fibonacci.series(9)
       test/fibonacci_test.exs:6: (test)

Finished in 0.03 seconds
1 test, 1 failure
```

---

# Test-Driven Development

```elixir
defmodule Fibonacci do

def series(_length) do
    []
  end
end
```

---

# Test-Driven Development

```elixir
$ mix test
Compiling 1 file (.ex)

1) test returns list of specified length (FibonacciTest)
     test/fibonacci_test.exs:5
     Assertion with == failed
     code: Fibonacci.series(8) == [0, 1, 1, 2, 3, 5, 8, 13]
     lhs:  []
     rhs:  [0, 1, 1, 2, 3, 5, 8, 13]
     stacktrace:
       test/fibonacci_test.exs:6: (test)

Finished in 0.03 seconds
1 test, 1 failure
```

---

```elixir
defmodule Fibonacci do

def series(0), do: []
  def series(1), do: [0]
  def series(length) do
    [number_at_position(length) | Enum.reverse(series(length - 1))]
    |> Enum.reverse
  end

defp number_at_position(1), do: 0
  defp number_at_position(2), do: 1
  defp number_at_position(number) do
    number_at_position(number - 2) + number_at_position(number - 1)
  end
end
```

---

# Green!

```shell
$ mix test
.

Finished in 0.04 seconds
1 test, 0 failures
```

---