Code Designing

Reference: See reference.md for complete design principles and examples.

Ready to implement? Use @testing skill for test structure.

Scan codebase structure:

• Vertical slicing: internal/feature/{handler,service,repository,models}.go
• Horizontal layering: internal/{handlers,services,domain}/feature.go

Always ask user approval with options:

• Option A: Vertical slice (recommended for cohesion/maintainability)
• Option B: Match existing pattern (if time-constrained)
• Acknowledge: Time pressure, team decisions, consistency needs are valid

If migration needed, create/update docs/architecture/vertical-slice-migration.md:

# Vertical Slice Migration Plan
## Current State: [horizontal/mixed]
## Target: Vertical slices in internal/[feature]/
## Strategy: New features vertical, migrate existing incrementally
## Progress: [x] [new-feature] (this PR), [ ] existing features

See reference.md section #3 for detailed patterns.

If yes to any → Consider creating a type

// Validating constructor func NewTypeName(input underlyingType) (TypeName, error) { // Validate input if /* validation fails */ { return zero, errors.New("why it failed") } return TypeName(input), nil }

// Methods on type (if behavior needed) func (t TypeName) SomeMethod() result { // Type-specific logic }

</design_self_validating_types>

<plan_package_structure>
- **Vertical slices**: Group by feature, not layer
- Each feature gets its own package
- Within package: separate by role (service, repository, handler)

Good structure:

user/ ├── user.go # Domain types ├── service.go # Business logic ├── repository.go # Persistence └── handler.go # HTTP/API

Bad structure:

domain/user.go services/user_service.go repository/user_repository.go

</plan_package_structure>

<design_orchestrating_types>
For types that coordinate others:
- Make fields private
- Validate dependencies in constructor
- No nil checks in methods (constructor guarantees validity)

```go
type Service struct {
    repo        Repository  // private
    notifier    Notifier    // private
}

func NewService(repo Repository, notifier Notifier) (*Service, error) {
    if repo == nil {
        return nil, errors.New("repo required")
    }
    if notifier == nil {
        return nil, errors.New("notifier required")
    }
    return &Service{
        repo:     repo,
        notifier: notifier,
    }, nil
}

// Methods can trust fields are valid
func (s *Service) DoSomething() error {
    // No nil checks needed
    return s.repo.Save(...)
}

DESIGN PLAN

Feature: [Feature Name]

Core Domain Types:
- UserID (string) - Self-validating, prevents empty IDs
- Email (string) - Self-validating, RFC 5322 validation
- Age (int) - Self-validating, range 0-150

Orchestrating Types:
- UserService - Coordinates user operations
   Dependencies: Repository, Notifier
   Methods: CreateUser, GetUser, UpdateUser

Package Structure:
user/
  ├── user.go          # UserID, Email, Age, User
  ├── service.go       # UserService
  ├── repository.go    # Repository interface + implementations
  ├── notifier.go      # Notifier interface + implementations
  └── handler.go       # HTTP handlers

Design Decisions:
- UserID is custom type to prevent passing empty/invalid IDs
- Email validation centralized in NewEmail constructor
- Vertical slice keeps all user logic in one package
- Repository as interface allows multiple backends (Postgres, in-memory for tests)

Integration Points:
- Consumed by: HTTP API (/users endpoints)
- Depends on: Database, Email service
- Events: UserCreated event published after creation

Next Steps:
1. Create types with validating constructors
2. Write unit tests for each type
3. Implement UserService
4. Write integration tests

Ready to implement? Use @testing skill for test structure.

Before writing code, ask:

• Can logic be moved into smaller custom types?
• Is this type designed around intent and behavior?
• Have I avoided primitive obsession?
• Is validation in the right place (constructor)?
• Does this follow vertical slice architecture?

Only after satisfactory answers, proceed to implementation.

See reference.md for complete design principles and examples.

• Architecture pattern analyzed (vertical/horizontal/mixed)
• Core domain types identified with validation rules
• Self-validating type design documented
• Package structure follows vertical slice pattern
• Design decisions documented with rationale
• Pre-code review questions answered satisfactorily
• Design plan output presented to user