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

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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

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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.

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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