My Xcx Rules

Overview

Elixir and Phoenix Best Practices

Based on Dave Thomas' (PragDave) coding philosophy Important: always use lates versions of packages and libraries, including Phoenix.

Core Principles

• Domain-Driven Design: Organize code around business domains, not technical layers
• Functional Core, Imperative Shell: Pure domain logic with side effects at boundaries
• Explicit Over Implicit: Prefer clarity over magic
• Composition Over Inheritance: Build systems from small, focused components
• Single Responsibility: Each module and function should do one thing well
• Easy to Change: Design for maintainability and future change
• Fail Fast: Detect and handle errors as early as possible
• YAGNI: Don't build features until they're needed

Project Structure

• Context-Based Organization: Use Phoenix contexts to define domain boundaries lib/my_app/ accounts/ # User management domain billing/ # Payment processing domain catalog/ # Product catalog domain

• API/Implementation Separation: Public API modules delegate to implementation modules

  In MyApp.Accounts (API module)

  defdelegate create_user(attrs), to: MyApp.Accounts.UserCreator

• Boundary Enforcement: Use tools like NimbleOptions to validate inputs at boundaries

Coding Patterns

• Pattern Matching: Use pattern matching in function heads for control flow

• Railway-Oriented Programming: Chain operations with 'with' for elegant error handling

  with {:ok, user} <- find_user(id), {:ok, updated} <- update_user(user, attrs) do {:ok, updated} end

• Type Specifications: Add typespecs to all public functions

  @spec create_user(user_attrs()) :: {:ok, User.t()} | {:error, Changeset.t()}

• Immutable Data Transformations: Return new state rather than modifying existing state

• Data Validation: Validate data at boundaries using Ecto.Changeset even outside of database contexts

  def validate_attrs(attrs) do {%{}, %{name: :string, email: :string}} |> Ecto.Changeset.cast(attrs, [:name, :email]) |> Ecto.Changeset.validate_required([:name, :email]) |> Ecto.Changeset.validate_format(:email, ~r/@/) end

• Result Tuples: Return tagged tuples like '{:ok, result}' or '{:error, reason}' for operations that can fail

Process Design

• GenServer for State: Use GenServers for stateful processes
• Supervision Trees: Design proper supervision hierarchies
• Registry Pattern: Use Registry for dynamic process lookup
• Task.Supervisor: Use for concurrent, potentially failing operations
• Process Isolation: Design processes to crash independently without affecting the whole system
• Let It Crash: Embrace the "let it crash" philosophy with proper supervision

Phoenix Best Practices

• LiveView-First: Use LiveView as the primary UI technology
• Function Components: Use function components for reusable UI elements
• PubSub for Real-time: Use Phoenix PubSub for real-time features
• Context Boundaries: Respect context boundaries in controllers and LiveViews
• Thin Controllers: Keep controllers thin, delegating business logic to contexts
• Security First: Always consider security implications (CSRF, XSS, etc.)

Testing Strategies

• Test Public APIs: Focus on testing public context APIs
• Mox for Dependencies: Use Mox for mocking external dependencies
• Property-Based Testing: Use StreamData for property-based tests
• Test Factories: Use ExMachina for test data creation
• Test Readability: Write tests that serve as documentation
• Arrange-Act-Assert: Structure tests with clear setup, action, and verification phases

HTTP and API Integration

• Req for HTTP Clients: Use Req instead of HTTPoison or Tesla
• Behaviours for API Clients: Define behaviours for API clients to allow easy mocking
• Error Handling: Handle network failures and unexpected responses gracefully
• Timeouts: Always set appropriate timeouts for external calls
• Circuit Breakers: Use circuit breakers for critical external services

Naming Conventions

• Snake Case: For variables and functions ('create_user')
• Verb-First Functions: Start function names with verbs ('create_user', not 'user_create')
• Plural for Collections: Use plural for collections ('users', not 'user')
• Consistent Terminology: Use consistent terms throughout the codebase
• Intention-Revealing Names: Choose names that reveal intent, not implementation

Documentation and Quality

• Document Public Functions: Add '@doc' to all public functions
• Examples in Docs: Include examples in documentation
• Credo and Dialyzer: Use for static analysis and type checking
• Consistent Formatting: Use 'mix format' to maintain consistent code style
• Continuous Refactoring: Regularly improve code structure without changing behavior
• Comments: Write comments only when necessary. Describe why, not what it does.

Performance Considerations

• Avoid N+1 Queries: Use Ecto's preloading and joins
• Pagination: Paginate large result sets
• Background Jobs: Use Oban for background processing
• Measure First: Profile before optimizing
• Caching: Apply strategic caching where appropriate