Elixir

Elixir Project Rules

Agent Automation Commands

CRITICAL: Execute these commands after EVERY implementation (see AGENT_AUTOMATION module for full workflow).

# Complete quality check sequence:
mix format --check-formatted  # Format check
mix credo --strict        # Linting
mix dialyzer              # Type checking
mix test                  # All tests (100% pass)
mix test --cover          # Coverage (95%+ required)
mix compile --warnings-as-errors  # Build

# Security audit:
mix hex.audit             # Vulnerability scan
mix hex.outdated          # Check outdated deps

Elixir Configuration

CRITICAL: Use Elixir 1.16+ with OTP 26+.

• Version: Elixir 1.16+
• OTP: 26+
• Formatter: Built-in mix format
• Linter: Credo
• Type Checker: Dialyzer

mix.exs Requirements

defmodule YourProject.MixProject do
  use Mix.Project

  def project do
    [
      app: :your_project,
      version: "1.0.0",
      elixir: "~> 1.16",
      start_permanent: Mix.env() == :prod,
      deps: deps(),
      
      # Documentation
      name: "Your Project",
      source_url: "https://github.com/your-org/your-project",
      docs: [
        main: "readme",
        extras: ["README.md", "CHANGELOG.md"]
      ],
      
      # Testing
      test_coverage: [tool: ExCoveralls],
      preferred_cli_env: [
        coveralls: :test,
        "coveralls.detail": :test,
        "coveralls.post": :test,
        "coveralls.html": :test
      ],
      
      # Dialyzer
      dialyzer: [
        plt_add_apps: [:mix, :ex_unit],
        plt_file: {:no_warn, "priv/plts/dialyzer.plt"},
        flags: [:error_handling, :underspecs]
      ]
    ]
  end

  def application do
    [
      extra_applications: [:logger],
      mod: {YourProject.Application, []}
    ]
  end

  defp deps do
    [
      # Development & Testing
      {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
      {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false},
      {:excoveralls, "~> 0.18", only: :test},
      {:ex_doc, "~> 0.31", only: :dev, runtime: false}
    ]
  end
end

Code Quality Standards

Mandatory Quality Checks

CRITICAL: After implementing ANY feature, you MUST run these commands in order.

IMPORTANT: These commands MUST match your GitHub Actions workflows to prevent CI/CD failures!

# Pre-Commit Checklist (MUST match .github/workflows/*.yml)

# 1. Format check (matches workflow - use --check-formatted!)
mix format --check-formatted

# 2. Lint (MUST pass with no warnings - matches workflow)
mix credo --strict

# 3. Type check with Dialyzer (matches workflow)
mix dialyzer

# 4. Run all tests (MUST pass 100% - matches workflow)
mix test --cover

# 5. Check coverage (MUST meet threshold)
mix test --cover --export-coverage default
mix test.coverage

# If ANY fails: ❌ DO NOT COMMIT - Fix first!

If ANY of these fail, you MUST fix the issues before committing.

Why This Matters:

• CI/CD failures happen when local commands differ from workflows
• Example: Using mix format locally but mix format --check-formatted in CI = failure
• Example: Missing --cover flag = CI coverage failures

Formatting

• Use built-in mix format
• Configuration in .formatter.exs
• Format before committing: mix format

Example .formatter.exs:

[
  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
  line_length: 100
]

Linting

• Use Credo for code analysis
• Configuration in .credo.exs
• Must pass strict mode: mix credo --strict

Example .credo.exs:

%{
  configs: [
    %{
      name: "default",
      files: %{
        included: ["lib/", "test/"],
        excluded: [~r"/_build/", ~r"/deps/"]
      },
      strict: true,
      color: true,
      checks: %{
        enabled: [
          {Credo.Check.Readability.ModuleDoc, []},
          {Credo.Check.Design.AliasUsage, priority: :low}
        ]
      }
    }
  ]
}

Testing

• Framework: ExUnit (built-in)
• Location: test/ directory
• Coverage: ExCoveralls
• Coverage Threshold: 95%+

Example test structure:

defmodule YourProject.MyModuleTest do
  use ExUnit.Case, async: true
  
  doctest YourProject.MyModule

  describe "function_name/1" do
    test "handles valid input" do
      assert YourProject.MyModule.function_name("input") == {:ok, "result"}
    end

    test "returns error for invalid input" do
      assert YourProject.MyModule.function_name("") == {:error, :invalid_input}
    end
  end
end

Type Specifications

• Use @spec for all public functions
• Use @type for custom types
• Run Dialyzer regularly

Example:

defmodule YourProject.MyModule do
  @moduledoc """
  Documentation for MyModule.
  """

  @type result :: {:ok, String.t()} | {:error, atom()}

  @spec process(String.t()) :: result()
  def process(input) when is_binary(input) and input != "" do
    {:ok, String.upcase(input)}
  end

  def process(_), do: {:error, :invalid_input}
end

Documentation

• Use @moduledoc for module documentation
• Use @doc for function documentation
• Include examples with doctests
• Generate docs with mix docs

Example:

defmodule YourProject.MyModule do
  @moduledoc """
  Provides functionality for processing data.

  ## Examples

      iex> YourProject.MyModule.process("hello")
      {:ok, "HELLO"}
  """

  @doc """
  Processes the input string.

  Returns `{:ok, result}` on success or `{:error, reason}` on failure.

  ## Examples

      iex> YourProject.MyModule.process("test")
      {:ok, "TEST"}

      iex> YourProject.MyModule.process("")
      {:error, :invalid_input}
  """
  @spec process(String.t()) :: {:ok, String.t()} | {:error, atom()}
  def process(input) when is_binary(input) and input != "" do
    {:ok, String.upcase(input)}
  end

  def process(_), do: {:error, :invalid_input}
end

Project Structure

project/
├── mix.exs             # Project configuration
├── .formatter.exs      # Formatter configuration
├── .credo.exs          # Credo configuration
├── README.md           # Project overview (allowed in root)
├── CHANGELOG.md        # Version history (allowed in root)
├── LICENSE             # Project license (allowed in root)
├── lib/
│   ├── your_project.ex       # Main module
│   └── your_project/
│       ├── application.ex    # OTP application
│       └── ...
├── test/
│   ├── test_helper.exs      # Test configuration
│   └── your_project/
│       └── ...
├── config/
│   ├── config.exs           # General config
│   ├── dev.exs              # Development config
│   ├── test.exs             # Test config
│   └── prod.exs             # Production config
├── priv/                    # Private assets
└── docs/                    # Project documentation

Error Handling

• Use tagged tuples: {:ok, value} and {:error, reason}
• Use with for multiple operations
• Create custom error modules when needed

Example:

defmodule YourProject.Errors do
  defmodule ValidationError do
    defexception [:message, :field]
  end
end

defmodule YourProject.MyModule do
  alias YourProject.Errors.ValidationError

  def validate(data) do
    with {:ok, cleaned} <- clean_data(data),
         {:ok, validated} <- check_format(cleaned) do
      {:ok, validated}
    else
      {:error, :empty} -> 
        raise ValidationError, message: "Data cannot be empty", field: :data
      
      {:error, reason} -> 
        {:error, reason}
    end
  end
end

OTP Best Practices

• Use Supervisors for fault tolerance
• Implement GenServers for stateful processes
• Use Task for concurrent operations

Example Supervisor:

defmodule YourProject.Application do
  use Application

  @impl true
  def start(_type, _args) do
    children = [
      {YourProject.MyWorker, []},
      {Task.Supervisor, name: YourProject.TaskSupervisor}
    ]

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

CI/CD Requirements

Must include GitHub Actions workflows for:

1. Testing (elixir-test.yml):

   • Test on ubuntu-latest
   • Test on Elixir 1.16, 1.17
   • Test on OTP 26, 27
   • Upload coverage reports

2. Linting (elixir-lint.yml):

   • Format check: mix format --check-formatted
   • Credo: mix credo --strict
   • Dialyzer: mix dialyzer

Package Publication

Publishing to Hex.pm

Prerequisites:

1. Create account at https://hex.pm
2. Generate API key: mix hex.user auth
3. Add HEX_API_KEY to GitHub repository secrets

mix.exs Configuration:

def project do
  [
    app: :your_package,
    version: "1.0.0",
    elixir: "~> 1.16",
    description: "A short description of your package",
    package: package(),
    docs: docs()
  ]
end

defp package do
  [
    name: :your_package,
    files: ~w(lib .formatter.exs mix.exs README.md LICENSE CHANGELOG.md),
    licenses: ["MIT"],
    links: %{
      "GitHub" => "https://github.com/your-org/your-package",
      "Changelog" => "https://github.com/your-org/your-package/blob/main/CHANGELOG.md"
    },
    maintainers: ["Your Name"]
  ]
end

defp docs do
  [
    main: "readme",
    extras: ["README.md", "CHANGELOG.md"],
    source_url: "https://github.com/your-org/your-package"
  ]
end

Publishing Workflow:

1. Update version in mix.exs

2. Update CHANGELOG.md

3. Run quality checks:

   mix format
   mix credo --strict
   mix dialyzer
   mix test
   mix coveralls
   

4. Build docs: mix docs

5. Build package: mix hex.build

6. Create git tag: git tag v1.0.0 && git push --tags

7. GitHub Actions automatically publishes to Hex

8. Or manual publish: mix hex.publish

Publishing Checklist:

• ✅ All tests passing (mix test)
• ✅ Coverage meets threshold (mix coveralls)
• ✅ Credo passes strict mode (mix credo --strict)
• ✅ Dialyzer passes (mix dialyzer)
• ✅ Code formatted (mix format --check-formatted)
• ✅ Version updated in mix.exs
• ✅ CHANGELOG.md updated
• ✅ README.md up to date
• ✅ LICENSE file present
• ✅ Documentation builds (mix docs)
• ✅ Package metadata complete in mix.exs
• ✅ Verify with mix hex.build

Semantic Versioning:

Follow SemVer:

• MAJOR: Breaking API changes
• MINOR: New features (backwards compatible)
• PATCH: Bug fixes (backwards compatible)

Documentation:

HexDocs automatically generates documentation from your code:

• Published at: https://hexdocs.pm/your_package
• Updated automatically when publishing to Hex