feat: add hello world function with comprehensive tests

PRPs/features/completed/hello-world-function.md

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+# Feature: Hello World Function
+
+## Feature Description
+
+Create a simple hello world function in a new file called `hello_world_test_from_archon.py`. The function should accept a name parameter, print a personalized greeting message to stdout, and return the greeting string for programmatic use. This is a demonstration feature that follows Python 3.12+ best practices with full type safety, Google-style docstrings, and comprehensive testing.
+
+## User Story
+
+As a developer
+I want to have a simple hello world function with proper type hints and documentation
+So that I can use it as a reference implementation for creating well-documented, type-safe Python functions in this project
+
+## Problem Statement
+
+The project needs a reference implementation demonstrating:
+- Python 3.12+ type hints and modern syntax
+- Google-style docstrings following project conventions
+- Proper testing structure with unit tests
+- Compliance with project linting and type checking requirements
+- KISS principle: simple, maintainable code
+
+## Solution Statement
+
+Implement a standalone module `hello_world_test_from_archon.py` containing a `greet` function that:
+- Accepts an optional name parameter (defaults to "World")
+- Prints a greeting message to stdout
+- Returns the greeting string for programmatic use
+- Includes full type annotations using Python 3.12 syntax
+- Has a comprehensive Google-style docstring
+- Includes example usage in a `__main__` block
+- Has corresponding unit tests in the test suite
+
+## Relevant Files
+
+### Existing Files for Reference
+
+- **CLAUDE.md** (lines 1-330) - Project principles, coding standards, type safety requirements, testing conventions
+  - Relevant because: Defines KISS/YAGNI principles, type safety as core rule, Google-style docstring requirements
+
+- **pyproject.toml** (entire file) - Project configuration for linting, type checking, and testing
+  - Relevant because: Shows ruff, mypy, and pytest configuration that the new code must pass
+
+- **dylan/utility_library/shared/exit_command.py** - Example of simple utility functions with proper documentation
+  - Relevant because: Demonstrates Google-style docstrings, type hints, and function design patterns used in the project
+
+- **dylan/utility_library/shared/tests/test_exit_command.py** - Example test structure
+  - Relevant because: Shows testing patterns, pytest conventions, and how to structure unit tests
+
+### New Files
+
+- **hello_world_test_from_archon.py** (new file at project root) - Main implementation
+  - Contains the `greet()` function with type hints and docstring
+  - Includes `__main__` block for direct execution
+
+- **dylan/tests/test_hello_world_test_from_archon.py** (new test file) - Unit tests
+  - Tests default behavior (no name provided)
+  - Tests custom name behavior
+  - Tests return value correctness
+  - Tests output formatting
+
+## Relevant research docstring
+
+Use these documentation files and links to help with understanding the technology to use:
+
+- [Google Python Style Guide - Docstrings](https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings)
+  - Section 3.8: Comments and Docstrings
+  - Shows proper format for function docstrings with Args, Returns, and Examples sections
+
+- [Python Type Hints Documentation](https://docs.python.org/3.12/library/typing.html)
+  - Type hints reference for Python 3.12
+  - Modern syntax using `str | None` instead of `Optional[str]`
+
+- [Real Python - Documenting Python Code](https://realpython.com/documenting-python-code/)
+  - Complete guide to Python documentation
+  - Best practices for docstrings and type hints working together
+
+- [Python 3.12 Type Hint Improvements](https://www.andy-pearce.com/blog/posts/2023/Dec/whats-new-in-python-312-type-hint-improvements/)
+  - New features in Python 3.12 for type hints
+  - Use of modern union syntax with `|`
+
+## Implementation Plan
+
+### Phase 1: Foundation
+
+Create the basic function implementation following Python 3.12 best practices:
+- Define function signature with complete type annotations
+- Write comprehensive Google-style docstring
+- Implement simple logic following KISS principle
+- Add `__main__` block for direct execution demonstration
+
+### Phase 2: Core Implementation
+
+Implement the `greet()` function in `hello_world_test_from_archon.py`:
+- Accept optional `name` parameter with default value
+- Use type hints: `def greet(name: str = "World") -> str:`
+- Print formatted greeting to stdout
+- Return the greeting string
+- Include docstring with Args, Returns, and Examples sections
+
+### Phase 3: Testing and Validation
+
+Create comprehensive unit tests and validate code quality:
+- Write unit tests in `dylan/tests/test_hello_world_test_from_archon.py`
+- Test default behavior, custom names, return values, and output
+- Run linters (ruff) and type checker (mypy)
+- Verify all tests pass with zero regressions
+
+## Step by Step Tasks
+
+### 1. Create the main module file
+
+- Create `hello_world_test_from_archon.py` at the project root
+- Add module-level docstring explaining the purpose
+- Define the `greet()` function with type hints
+- Implement function logic: print and return greeting
+- Add comprehensive Google-style docstring with Args, Returns, and Examples
+- Add `__main__` block for demonstrating usage
+- Follow Python 3.12 modern syntax (`str` type hints, f-strings)
+
+### 2. Create the test file
+
+- Create `dylan/tests/test_hello_world_test_from_archon.py`
+- Import pytest and the `greet` function
+- Mark tests with `@pytest.mark.unit` decorator
+- Write test for default behavior: `test_greet_default()`
+- Write test for custom name: `test_greet_custom_name()`
+- Write test for return value: `test_greet_return_value()`
+- Write test for output verification using `capsys` fixture
+- Follow existing test patterns from `test_exit_command.py`
+
+### 3. Run linting checks
+
+- Execute `uv run ruff check hello_world_test_from_archon.py`
+- Execute `uv run ruff check dylan/tests/test_hello_world_test_from_archon.py`
+- Fix any linting issues (naming, formatting, imports)
+- Ensure line length is within 100 characters
+
+### 4. Run type checking
+
+- Execute `uv run mypy hello_world_test_from_archon.py --strict`
+- Execute `uv run mypy dylan/tests/test_hello_world_test_from_archon.py`
+- Verify no type errors are reported
+- Ensure all functions have complete type annotations
+
+### 5. Run unit tests
+
+- Execute `uv run pytest dylan/tests/test_hello_world_test_from_archon.py -v`
+- Verify all tests pass
+- Check test coverage includes all function paths
+- Confirm zero test failures or errors
+
+### 6. Run full validation suite
+
+- Execute all validation commands listed below
+- Verify module can be imported successfully
+- Test direct execution: `uv run python hello_world_test_from_archon.py`
+- Test with custom name: `uv run python hello_world_test_from_archon.py "Claude"`
+- Confirm zero regressions in existing tests
+
+## Testing Strategy
+
+See `CLAUDE.md` for complete testing requirements. Every file in `dylan/` must have a corresponding test file in `dylan/tests/`.
+
+### Unit Tests
+
+All unit tests should be marked with `@pytest.mark.unit` decorator and placed in `dylan/tests/test_hello_world_test_from_archon.py`:
+
+1. **test_greet_default()** - Verify function works with default "World" parameter
+   - Assert return value is "Hello, World!"
+   - Verify correct output to stdout
+
+2. **test_greet_custom_name()** - Verify function works with custom name
+   - Test with single name: "Alice"
+   - Assert return value is "Hello, Alice!"
+   - Verify correct output to stdout
+
+3. **test_greet_return_value()** - Verify return type is string
+   - Assert return value is of type `str`
+   - Verify return value matches expected format
+
+4. **test_greet_empty_string()** - Edge case: empty string as name
+   - Test with empty string: `greet("")`
+   - Assert return value is "Hello, !"
+
+5. **test_greet_special_characters()** - Edge case: name with special characters
+   - Test with name containing unicode: "José"
+   - Assert proper handling of special characters
+
+### Integration Tests
+
+Not applicable for this simple standalone function. No integration with other components required.
+
+### Edge Cases
+
+- Empty string as name parameter
+- Name with unicode/special characters (émojis, accents)
+- Very long name strings (test string length limits)
+- Whitespace-only name strings
+- Name with newline or tab characters
+
+## Acceptance Criteria
+
+1. **Functionality**
+   - Function accepts optional name parameter with default "World"
+   - Function prints greeting message to stdout
+   - Function returns greeting string
+   - Module can be executed directly via `__main__` block
+
+2. **Code Quality**
+   - All functions have complete type annotations (passes `mypy --strict`)
+   - All functions have Google-style docstrings
+   - Code passes all ruff linting checks
+   - Code follows KISS principle - no unnecessary complexity
+
+3. **Testing**
+   - Unit tests achieve 100% code coverage
+   - All tests pass with zero failures
+   - Tests are properly marked with `@pytest.mark.unit`
+   - Tests follow existing project patterns
+
+4. **Documentation**
+   - Docstring includes clear description
+   - Docstring documents Args section
+   - Docstring documents Returns section
+   - Docstring includes Examples section
+   - Module docstring explains purpose
+
+## Validation Commands
+
+Execute every command to validate the feature works correctly with zero regressions.
+
+**Linting checks:**
+```bash
+uv run ruff check hello_world_test_from_archon.py
+uv run ruff check dylan/tests/test_hello_world_test_from_archon.py
+```
+
+**Type checking:**
+```bash
+uv run mypy hello_world_test_from_archon.py --strict
+uv run mypy dylan/tests/test_hello_world_test_from_archon.py
+```
+
+**Unit tests:**
+```bash
+uv run pytest dylan/tests/test_hello_world_test_from_archon.py -v -m unit
+```
+
+**Full test suite (verify zero regressions):**
+```bash
+uv run pytest dylan/tests/ -v
+```
+
+**Manual execution tests:**
+```bash
+# Test default behavior
+uv run python hello_world_test_from_archon.py
+
+# Test module import
+uv run python -c "from hello_world_test_from_archon import greet; print(greet('Claude'))"
+```
+
+**Required validation commands:**
+- `uv run ruff check hello_world_test_from_archon.py` - Lint check must pass
+- `uv run mypy hello_world_test_from_archon.py --strict` - Type check must pass
+- `uv run pytest dylan/tests/test_hello_world_test_from_archon.py -v` - All tests must pass
+
+## Notes
+
+### Design Decisions
+
+- **File location**: Placed at project root for easy access and demonstration
+- **Function name**: `greet()` is more descriptive than `hello_world()`
+- **Return value**: Function both prints and returns for maximum flexibility
+- **Default parameter**: "World" follows traditional hello world convention
+
+### Future Considerations
+
+This is a simple demonstration feature. Potential enhancements could include:
+- Support for multiple languages/localization
+- Customizable greeting messages beyond "Hello"
+- Integration with Dylan CLI as a subcommand
+- Extended greeting formats (time-based greetings, etc.)
+
+However, following YAGNI principle, these features should only be added if explicitly needed.
+
+### Additional Context
+
+This feature demonstrates the project's commitment to:
+- Type safety as a core rule (strict mypy checking)
+- Comprehensive documentation (Google-style docstrings)
+- Thorough testing (unit tests for all code paths)
+- Simple, maintainable code (KISS principle)
+- Modern Python practices (Python 3.12+ syntax)

dylan/tests/test_hello_world_test_from_archon.py

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+"""Tests for hello_world_test_from_archon module."""
+
+import pytest
+
+from hello_world_test_from_archon import greet
+
+
+def test_greet_default() -> None:
+    """Test greet function with default World parameter."""
+    result = greet()
+    assert result == "Hello, World!"
+    assert isinstance(result, str)
+
+
+def test_greet_custom_name() -> None:
+    """Test greet function with custom name."""
+    result = greet("Alice")
+    assert result == "Hello, Alice!"
+    assert isinstance(result, str)
+
+
+def test_greet_return_value() -> None:
+    """Test that greet returns correct string type and format."""
+    result = greet("Bob")
+    assert isinstance(result, str)
+    assert result == "Hello, Bob!"
+    assert result.startswith("Hello, ")
+    assert result.endswith("!")
+
+
+def test_greet_output(capsys: pytest.CaptureFixture[str]) -> None:
+    """Test that greet prints to stdout correctly."""
+    greet("Charlie")
+    captured = capsys.readouterr()
+    assert captured.out == "Hello, Charlie!\n"
+
+
+def test_greet_empty_string() -> None:
+    """Test greet with empty string as name."""
+    result = greet("")
+    assert result == "Hello, !"
+    assert isinstance(result, str)
+
+
+def test_greet_special_characters() -> None:
+    """Test greet with special characters and unicode."""
+    result = greet("José")
+    assert result == "Hello, José!"
+    assert isinstance(result, str)
+
+
+def test_greet_with_spaces() -> None:
+    """Test greet with name containing spaces."""
+    result = greet("Jane Doe")
+    assert result == "Hello, Jane Doe!"
+    assert isinstance(result, str)
+
+
+def test_greet_unicode_emoji() -> None:
+    """Test greet with emoji characters."""
+    result = greet("Alice 👋")
+    assert result == "Hello, Alice 👋!"
+    assert isinstance(result, str)