Skill: TDD Implementation

When to use this skill

• Implementing new features
• Major refactoring
• Complex bug fixes

Available templates

Template	Usage
templates/test_module.md	Test module structure
templates/module_structure.md	DDD structure for new module

TDD Workflow (Test-Driven Development)

Phase 1: Understand

1. Read existing documentation

   • GLOBAL_APP_DESCRIPTION.md for global context
   • docs/STRATEGIES.md if related to trading

2. Identify affected modules

   • Which DDD layer? (domain/application/infrastructure)
   • Which existing files will be impacted?

3. Verify architecture

   domain/         → Pure business logic, no I/O
   application/    → Orchestration, services, use cases
   infrastructure/ → I/O, external APIs, persistence
   

Phase 2: Write tests FIRST

Fundamental rule: Tests are written BEFORE implementation code.

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_new_feature_basic_case() {
        // Arrange
        let input = ...;
        
        // Act
        let result = new_feature(input);
        
        // Assert
        assert_eq!(result, expected);
    }

    #[test]
    fn test_new_feature_edge_case() {
        // Test edge cases
    }

    #[test]
    fn test_new_feature_error_case() {
        // Test error cases
    }
}

Tests MUST fail at this stage (red phase).

Phase 3: Implement

1. Minimal code to make tests pass
2. Respect DDD architecture
3. No .unwrap() in production code
4. Use Decimal for amounts

// ❌ Avoid
fn process(value: f64) -> f64 {
    some_operation(value).unwrap()
}

// ✅ Prefer
fn process(value: Decimal) -> Result<Decimal, ProcessError> {
    some_operation(value).map_err(ProcessError::from)
}

Phase 4: Refactor

1. Clean the code

   • Remove duplicate code
   • Remove commented-out code and obsolete comments
   • Improve variable names
   • Simplify complex expressions

2. Add documentation

   • Rustdoc on public functions
   • Comments for complex logic

3. Verify with clippy

   cargo clippy --all-targets -- -D warnings
   

Phase 5: Validate

1. Invoke the testing skill

   • cargo fmt
   • cargo clippy
   • cargo test

2. Invoke the documentation skill

   • Update GLOBAL_APP_DESCRIPTION.md
   • Add entry in GLOBAL_APP_DESCRIPTION_VERSIONS.md
   • Increment version

Complete example

// 1. First the test
#[test]
fn test_calculate_position_size() {
    let capital = Decimal::from(10000);
    let risk_pct = Decimal::from_str("0.02").unwrap();
    let stop_loss = Decimal::from_str("0.05").unwrap();
    
    let size = calculate_position_size(capital, risk_pct, stop_loss);
    
    assert_eq!(size, Decimal::from(4000)); // 2% risk, 5% stop = 40% position
}

// 2. Then the implementation
pub fn calculate_position_size(
    capital: Decimal,
    risk_percentage: Decimal,
    stop_loss_percentage: Decimal,
) -> Decimal {
    let risk_amount = capital * risk_percentage;
    risk_amount / stop_loss_percentage
}

Anti-patterns to avoid

Anti-pattern	Problem	Solution
Tests after code	Confirmation bias	Strict TDD
.unwrap() everywhere	Panics in production	Error handling
f64 for money	Rounding errors	Decimal
Tests without assertions	False positives	At least one assertion per test
Modify tests to pass	Hides bugs	Fix the code
Commented-out code	Clutters codebase	Remove it (use git history)