MCPInstall

elixir-antipatterns

by gentleman-programmingCode Quality & Workflow

Core catalog of 8 critical Elixir/Phoenix anti-patterns covering error handling, separation of concerns, Ecto queries, and testing. Trigger: During Elixir code review, refactoring sessions, or when writing Phoenix/Ecto code.

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Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/gentleman-programming/gentleman-skills --skill elixir-antipatterns
  1. 01

    Install

    Run the command in your project. The CLI asks which agents to install for.

  2. 02

    Describe the task

    Ask your agent for something this skill covers. It loads elixir-antipatterns automatically when relevant.

  3. 03

    Review the result

    The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

Elixir Anti-Patterns

Critical anti-patterns that compromise robustness and maintainability in Elixir/Phoenix applications.

Complement with: mix format and Credo for style enforcement
Extended reference: See EXTENDED.md for 40+ patterns and deep-dive examples

When to Use

Topics: Error handling (3 patterns) • Architecture (2 patterns) • Performance (2 patterns) • Testing (1 pattern)

Load this skill when:

  • Writing Elixir modules and functions
  • Working with Phoenix Framework (Controllers, LiveView)
  • Building Ecto schemas and database queries
  • Implementing BEAM concurrency (Task, GenServer)
  • Handling errors with tagged tuples
  • Writing tests with ExUnit

Critical Patterns

Quick reference to the 8 core patterns this skill enforces:

  1. Tagged Tuples: Return {:ok, value} | {:error, reason} instead of nil or exceptions
  2. Explicit @spec: Document error cases in function signatures
  3. Context Separation: Business logic in contexts, not LiveView
  4. Preload Associations: Use Repo.preload/2 to avoid N+1 queries
  5. with Arrow Binding: Use <- for all failable operations in with
  6. Database Indexes: Index frequently queried columns
  7. Test Assertions: Every test must assert expected behavior
  8. Cohesive Functions: Group with chains >4 steps into functions

See ## Anti-Patterns section below for detailed ❌ BAD / ✅ CORRECT code examples.

Code Examples

Example 1: Error Handling with Tagged Tuples

# ✅ CORRECT - Errors as values, explicit in @spec
defmodule UserService do
  @spec fetch_user(String.t()) :: {:ok, User.t()} | {:error, :not_found}
  def fetch_user(id) do
    case Repo.get(User, id) do
      nil -> {:error, :not_found}
      user -> {:ok, user}
    end
  end
end

# ❌ BAD - Exceptions for business errors
def fetch_user(id) do
  Repo.get(User, id) || raise "User not found"
end

Example 2: Phoenix LiveView with Context Separation

Architecture Layers:
  User Request → LiveView (UI only) → Context (business logic) → Schema/Repo (data)
               ↓                    ↓                           ↓
           handle_event()     Accounts.create_user()      Repo.insert()

# ✅ CORRECT - Thin LiveView, logic in context
defmodule MyAppWeb.UserLive.Index do
  use MyAppWeb, :live_view
  
  def handle_event("create", params, socket) do
    case Accounts.create_user(params) do
      {:ok, user} -> {:noreply, redirect(socket, to: ~p"/users/#{user}")}
      {:error, changeset} -> {:noreply, assign(socket, changeset: changeset)}
    end
  end
end

# ❌ BAD - Business logic in LiveView
def handle_event("create", %{"user" => params}, socket) do
  if String.length(params["name"]) < 3 do
    {:noreply, put_flash(socket, :error, "Too short")}
  else
    case Repo.insert(User.changeset(%User{}, params)) do
      {:ok, user} -> send_email(user); redirect(socket)
    end
  end
end

Example 3: Ecto N+1 Query Optimization

# ✅ CORRECT - Preload associations (2 queries total)
users = User |> Repo.all() |> Repo.preload(:posts)
Enum.map(users, fn user -> process(user, user.posts) end)

# Note: For complex filtering (e.g., WHERE posts.status = 'published'),
# use join + preload in the query itself. See EXTENDED.md for advanced patterns.

# ❌ BAD - Query in loop (101 queries for 100 users)
users = Repo.all(User)
Enum.map(users, fn user ->
  posts = Repo.all(from p in Post, where: p.user_id == ^user.id)
  {user, posts}
end)

Anti-Patterns

Error Management

Don't: Use raise for Business Errors

# ❌ BAD
def fetch_user(id) do
  Repo.get(User, id) || raise "User not found"
end

# ✅ CORRECT
@spec fetch_user(String.t()) :: {:ok, User.t()} | {:error, :not_found}
def fetch_user(id) do
  case Repo.get(User, id) do
    nil -> {:error, :not_found}
    user -> {:ok, user}
  end
end

Why: @spec documents errors, pattern matching forces explicit handling.

Don't: Return nil for Errors

# ❌ BAD - No context on failure
def find_user(email), do: Repo.get_by(User, email: email)

# ✅ CORRECT - Explicit error reason
@spec find_user(String.t()) :: {:ok, User.t()} | {:error, :not_found}
def find_user(email) do
  case Repo.get_by(User, email: email) do
    nil -> {:error, :not_found}
    user -> {:ok, user}
  end
end

Don't: Use = Inside with for Failable Operations

# ❌ BAD - Validate errors silenced
with {:ok, user} <- fetch_user(id),
     validated = validate(user),  # ← Doesn't check for {:error, _}
     {:ok, saved} <- save(validated) do
  {:ok, saved}
end

# ✅ CORRECT - All operations use <-
with {:ok, user} <- fetch_user(id),
     {:ok, validated} <- validate(user),
     {:ok, saved} <- save(validated) do
  {:ok, saved}
end

Architecture & Boundaries

Don't: Put Business Logic in LiveView

# ❌ BAD - Validation in view
def handle_event("create", %{"user" => params}, socket) do
  if String.length(params["name"]) < 3 do
    {:noreply, put_flash(socket, :error, "Too short")}
  else
    case Repo.insert(User.changeset(%User{}, params)) do
      {:ok, user} -> redirect(socket)
    end
  end
end

# ✅ CORRECT - Delegate to context
def handle_event("create", params, socket) do
  case Accounts.create_user(params) do
    {:ok, user} -> {:noreply, redirect(socket, to: ~p"/users/#{user}")}
    {:error, changeset} -> {:noreply, assign(socket, changeset: changeset)}
  end
end

Why: Contexts testable without Phoenix, logic reusable.

Don't: Chain More Than 4 Steps in with

# ❌ BAD - Too many responsibilities
with {:ok, a} <- step1(),
     {:ok, b} <- step2(a),
     {:ok, c} <- step3(b),
     {:ok, d} <- step4(c),
     {:ok, e} <- step5(d) do
  {:ok, e}
end

# ✅ CORRECT - Group into cohesive functions
with {:ok, validated} <- validate_and_fetch(id),
     {:ok, processed} <- process_business_rules(validated),
     {:ok, result} <- persist_and_notify(processed) do
  {:ok, result}
end

Data & Performance

Don't: Query Inside Loops (N+1)

# ❌ BAD - 101 queries for 100 users
users = Repo.all(User)
Enum.map(users, fn user ->
  posts = Repo.all(from p in Post, where: p.user_id == ^user.id)
end)

# ✅ CORRECT - 2 queries total
User |> Repo.all() |> Repo.preload(:posts)

Impact: 100 users with N+1 = 10 seconds vs 5ms with preload.

Don't: Query Without Indexes

# ❌ BAD - No index on frequently queried column
# Migration:
create table(:users) do
  add :email, :string
end

# ✅ CORRECT - Add index
create table(:users) do
  add :email, :string
end
create unique_index(:users, [:email])

Why: Full table scan on 1M+ rows vs instant index lookup.

Testing

Don't: Write Tests Without Assertions

# ❌ BAD - What's being tested?
test "creates user" do
  UserService.create_user(%{name: "Juan"})
end

# ✅ CORRECT - Assert expected behavior
test "creates user successfully" do
  assert {:ok, user} = UserService.create_user(%{name: "Juan"})
  assert user.name == "Juan"
end

Quick Reference

SituationAnti-PatternCorrect Pattern
Error handlingraise "Not found"{:error, :not_found}
Missing dataReturn nil{:error, :not_found}
Business logicIn LiveViewIn context modules
AssociationsEnum.map + Repo.getRepo.preload
with chainsvalidated = fn(){:ok, validated} <- fn()
Frequent queriesNo indexcreate index(:table, [:column])
TestingNo assertionsassert expected behavior
Complex logic6+ step withGroup into 3 functions

Resources

FAQ

What does the elixir-antipatterns skill do?

Core catalog of 8 critical Elixir/Phoenix anti-patterns covering error handling, separation of concerns, Ecto queries, and testing. Trigger: During Elixir code review, refactoring sessions, or when writing Phoenix/Ecto code.

How do I install elixir-antipatterns?

Run npx skills add https://github.com/gentleman-programming/gentleman-skills --skill elixir-antipatterns in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/elixir-antipatterns/ (personal) or .claude/skills/elixir-antipatterns/ (project).

Which AI agents can use elixir-antipatterns?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is elixir-antipatterns safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains elixir-antipatterns?

It is published by gentleman-programming in gentleman-programming/gentleman-skills. It has 191 recorded installs.

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