Elixir Expert

Expert guidance for Elixir programming, Phoenix framework, OTP, and building highly concurrent, fault-tolerant systems.

Core Concepts

Elixir Fundamentals

Functional programming
Pattern matching
Immutability
Pipe operator
Modules and functions
Structs and maps

OTP (Open Telecom Platform)

GenServer
Supervisors
Applications
Tasks
Agents
ETS (Erlang Term Storage)

Phoenix Framework

Contexts and schemas
LiveView
Channels (WebSockets)
Plug
Ecto (database)
Testing

Elixir Basics

# Pattern matching
{:ok, result} = {:ok, 42}
[head | tail] = [1, 2, 3, 4]

# Functions
defmodule Math do
  def add(a, b), do: a + b

  def factorial(0), do: 1
  def factorial(n) when n > 0, do: n * factorial(n - 1)
end

# Pipe operator
"hello"
|> String.upcase()
|> String.reverse()
# "OLLEH"

# Anonymous functions
add = fn a, b -> a + b end
add.(1, 2)  # 3

# Capture operator
add = &(&1 + &2)
Enum.map([1, 2, 3], &(&1 * 2))

# Structs
defmodule User do
  defstruct [:id, :name, :email, age: 0]
end

user = %User{id: "123", name: "John", email: "john@example.com"}

# Pattern matching with structs
%User{name: name} = user

# Maps
user_map = %{id: "123", name: "John", email: "john@example.com"}
%{name: name} = user_map

# Conditionals
if user.age >= 18 do
  "Adult"
else
  "Minor"
end

# Case
case user.role do
  :admin -> "Administrator"
  :user -> "Regular user"
  _ -> "Unknown"
end

# Cond
cond do
  user.age < 13 -> "Child"
  user.age < 18 -> "Teen"
  user.age < 65 -> "Adult"
  true -> "Senior"
end

# With
with {:ok, user} <- fetch_user(id),
     {:ok, posts} <- fetch_posts(user.id) do
  {:ok, user, posts}
else
  {:error, reason} -> {:error, reason}
end

GenServer

defmodule UserCache do
  use GenServer

  # Client API
  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, %{}, opts)
  end

  def get(server, key) do
    GenServer.call(server, {:get, key})
  end

  def put(server, key, value) do
    GenServer.cast(server, {:put, key, value})
  end

  # Server Callbacks
  @impl true
  def init(initial_state) do
    {:ok, initial_state}
  end

  @impl true
  def handle_call({:get, key}, _from, state) do
    {:reply, Map.get(state, key), state}
  end

  @impl true
  def handle_cast({:put, key, value}, state) do
    {:noreply, Map.put(state, key, value)}
  end

  @impl true
  def handle_info(:cleanup, state) do
    # Periodic cleanup
    {:noreply, state}
  end
end

# Usage
{:ok, cache} = UserCache.start_link()
UserCache.put(cache, :user1, %User{name: "John"})
UserCache.get(cache, :user1)

Supervisors

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    children = [
      # Database connection pool
      {MyApp.Repo, []},

      # GenServer workers
      {UserCache, name: UserCache},
      {SessionManager, name: SessionManager},

      # Task supervisor
      {Task.Supervisor, name: MyApp.TaskSupervisor},

      # Phoenix endpoint
      MyAppWeb.Endpoint
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

# Custom supervisor
defmodule WorkerSupervisor do
  use Supervisor

  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
  end

  @impl true
  def init(_init_arg) do
    children = [
      {Worker, id: :worker1, restart: :permanent},
      {Worker, id: :worker2, restart: :temporary},
      {Worker, id: :worker3, restart: :transient}
    ]

    Supervisor.init(children, strategy: :one_for_one)
  end
end

Ecto

# Schema
defmodule MyApp.Accounts.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :name, :string
    field :email, :string
    field :age, :integer
    field :role, Ecto.Enum, values: [:admin, :user, :guest]

    has_many :posts, MyApp.Blog.Post

    timestamps()
  end

  def changeset(user, attrs) do
    user
    |> cast(attrs, [:name, :email, :age, :role])
    |> validate_required([:name, :email])
    |> validate_format(:email, ~r/@/)
    |> validate_number(:age, greater_than: 0, less_than: 150)
    |> unique_constraint(:email)
  end
end

# Queries
import Ecto.Query

# Simple query
Repo.all(from u in User, where: u.age > 18)

# With preload
Repo.all(from u in User, preload: [:posts])

# Complex query
query =
  from u in User,
    join: p in assoc(u, :posts),
    where: u.age > 18,
    group_by: u.id,
    having: count(p.id) > 5,
    select: {u, count(p.id)}

Repo.all(query)

# Dynamic queries
defmodule UserQueries do
  import Ecto.Query

  def base_query, do: from(u in User)

  def filter_by_age(query, min_age) do
    from u in query, where: u.age >= ^min_age
  end

  def filter_by_role(query, role) do
    from u in query, where: u.role == ^role
  end

  def with_posts(query) do
    from u in query, preload: [:posts]
  end
end

# Usage
User
|> UserQueries.base_query()
|> UserQueries.filter_by_age(18)
|> UserQueries.filter_by_role(:admin)
|> UserQueries.with_posts()
|> Repo.all()

Phoenix LiveView

defmodule MyAppWeb.UserLive.Index do
  use MyAppWeb, :live_view

  alias MyApp.Accounts

  @impl true
  def mount(_params, _session, socket) do
    if connected?(socket) do
      Phoenix.PubSub.subscribe(MyApp.PubSub, "users")
    end

    {:ok, assign(socket, users: Accounts.list_users())}
  end

  @impl true
  def handle_event("delete", %{"id" => id}, socket) do
    user = Accounts.get_user!(id)
    {:ok, _} = Accounts.delete_user(user)

    {:noreply, assign(socket, users: Accounts.list_users())}
  end

  @impl true
  def handle_info({:user_created, user}, socket) do
    {:noreply, update(socket, :users, fn users -> [user | users] end)}
  end

  @impl true
  def render(assigns) do
    ~H"""
    <div>
      <h1>Users</h1>
      <table>
        <thead>
          <tr>
            <th>Name</th>
            <th>Email</th>
            <th>Actions</th>
          </tr>
        </thead>
        <tbody>
          <%= for user <- @users do %>
            <tr>
              <td><%= user.name %></td>
              <td><%= user.email %></td>
              <td>
                <button phx-click="delete" phx-value-id={user.id}>
                  Delete
                </button>
              </td>
            </tr>
          <% end %>
        </tbody>
      </table>
    </div>
    """
  end
end

Concurrency

# Tasks
task = Task.async(fn -> expensive_computation() end)
result = Task.await(task)

# Parallel processing
results =
  [1, 2, 3, 4, 5]
  |> Enum.map(&Task.async(fn -> process(&1) end))
  |> Enum.map(&Task.await/1)

# Task supervisor
Task.Supervisor.async_nolink(MyApp.TaskSupervisor, fn ->
  perform_background_job()
end)

# Agents
{:ok, agent} = Agent.start_link(fn -> %{} end)
Agent.update(agent, fn state -> Map.put(state, :key, "value") end)
Agent.get(agent, fn state -> Map.get(state, :key) end)

# ETS
table = :ets.new(:my_table, [:set, :public])
:ets.insert(table, {:key, "value"})
:ets.lookup(table, :key)

Best Practices

Elixir

Use pattern matching extensively
Leverage pipe operator
Write small, focused functions
Handle all pattern match cases
Use proper error handling
Follow naming conventions
Write documentation

OTP

Use GenServers for state
Implement supervisors properly
Design for failure
Keep processes focused
Use process monitoring
Implement timeouts
Log appropriately

Phoenix

Follow context boundaries
Use changesets for validation
Implement proper authentication
Handle errors gracefully
Test thoroughly
Use LiveView for interactivity
Optimize queries

Anti-Patterns

❌ Using mutable state ❌ Long-running GenServer calls ❌ Not supervising processes ❌ Ignoring errors ❌ Large modules ❌ N+1 query problems ❌ Not using pattern matching

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

Phoenix Framework

Resources

Elixir: https://elixir-lang.org/
Phoenix: https://www.phoenixframework.org/
Hex: https://hex.pm/
Elixir School: https://elixirschool.com/
Programming Elixir (book)

elixir-expert