Refactoring

Refactoring

Linter-driven refactoring patterns to reduce complexity and improve code quality.

When to Use

• Linter fails with complexity issues (cyclomatic, cognitive, maintainability)
• Code feels hard to read or maintain
• Functions are too long or deeply nested
• Automatically invoked by @linter-driven-development when linter fails

Learning Resources

Choose your learning path:

• Quick Start: Use the patterns below for common refactoring cases
• Complete Reference: See reference.md for full decision tree and all patterns
• Real-World Examples: See examples.md to learn the refactoring thought process
  • Example 1: Storifying and extracting a single leaf type
  • Example 2: Primitive obsession with multiple types and switch elimination

Refactoring Signals

Linter Failures

• Cyclomatic Complexity: Too many decision points → Extract functions, simplify logic
• Cognitive Complexity: Hard to understand → Storifying, reduce nesting
• Maintainability Index: Hard to maintain → Break into smaller pieces

Code Smells

• Functions > 50 LOC
• Nesting > 2 levels
• Mixed abstraction levels
• Unclear flow/purpose
• Primitive obsession

Workflow

1. Interpret Linter Output

Run task lintwithfix and analyze failures:

user/service.go:45:1: cyclomatic complexity 15 of func `CreateUser` is high (> 10)
user/handler.go:23:1: cognitive complexity 25 of func `HandleRequest` is high (> 15)

2. Diagnose Root Cause

For each failure, ask (see reference.md):

• Does this code read like a story? → Storifying needed
• Can this be broken into smaller pieces? → Extract functions/types
• Does logic run on primitives? → Primitive obsession
• Is function long due to switch statement? → Categorize and extract

3. Apply Refactoring Pattern

Choose appropriate pattern:

• Storifying: Extract helpers to clarify levels
• Extract Type: Move primitive logic to custom type
• Extract Function: Pull out complexity
• Early Returns: Reduce nesting
• Switch Extraction: Categorize cases

4. Verify Improvement

• Re-run linter
• Tests still pass
• Code more readable?

Refactoring Patterns

Pattern 1: Storifying (Mixed Abstractions)

Signal: Function mixes high-level steps with low-level details

// ❌ Before - Mixed abstractions
func ProcessOrder(order Order) error {
    // Validation
    if order.ID == "" {
        return errors.New("invalid")
    }

    // Low-level DB setup
    db, err := sql.Open("postgres", connStr)
    if err != nil { return err }
    defer db.Close()

    // SQL construction
    query := "INSERT INTO..."
    // ... many lines

    return nil
}

// ✅ After - Story-like
func ProcessOrder(order Order) error {
    if err := validateOrder(order); err != nil {
        return err
    }

    if err := saveToDatabase(order); err != nil {
        return err
    }

    return notifyCustomer(order)
}

func validateOrder(order Order) error { /* ... */ }
func saveToDatabase(order Order) error { /* ... */ }
func notifyCustomer(order Order) error { /* ... */ }

Pattern 2: Extract Type (Primitive Obsession)

Signal: Complex logic operating on primitives

// ❌ Before - Primitive obsession
func ValidatePort(port int) error {
    if port <= 0 || port >= 9000 {
        return errors.New("invalid port")
    }
    return nil
}

func GetServiceURL(host string, port int) string {
    return fmt.Sprintf("%s:%d", host, port)
}

// ✅ After - Custom type
type Port int

func NewPort(p int) (Port, error) {
    if p <= 0 || p >= 9000 {
        return 0, errors.New("invalid port")
    }
    return Port(p), nil
}

func (p Port) Int() int {
    return int(p)
}

type ServiceAddress struct {
    host string
    port Port
}

func (a ServiceAddress) URL() string {
    return fmt.Sprintf("%s:%d", a.host, a.port)
}

⚠️ Warning: Not Every Primitive Needs a Type!

Create types when: >1 meaningful methods, validation, complex logic, or controlled mutation DON'T when: One trivial method, good naming is enough, adds ceremony without benefit

// ❌ Over-abstraction
type IsValidFlag bool
func (f IsValidFlag) IsValid() bool { return bool(f) }

// ✅ Good naming or private fields
isValid bool  // Clear enough!

→ See Example 2 for complete case study.

Pattern 3: Extract Function (Long Functions)

Signal: Function > 50 LOC or multiple responsibilities

// ❌ Before - Long function
func CreateUser(data map[string]interface{}) error {
    // Validation (15 lines)
    // ...

    // Database operations (20 lines)
    // ...

    // Email notification (10 lines)
    // ...

    // Logging (5 lines)
    // ...

    return nil
}

// ✅ After - Extracted functions
func CreateUser(data map[string]interface{}) error {
    user, err := validateAndParseUser(data)
    if err != nil {
        return err
    }

    if err := saveUser(user); err != nil {
        return err
    }

    if err := sendWelcomeEmail(user); err != nil {
        return err
    }

    logUserCreation(user)
    return nil
}

Pattern 4: Early Returns (Deep Nesting)

Signal: Nesting > 2 levels

// ❌ Before - Deeply nested
func ProcessItem(item Item) error {
    if item.IsValid() {
        if item.IsReady() {
            if item.HasPermission() {
                // Process
                return nil
            } else {
                return errors.New("no permission")
            }
        } else {
            return errors.New("not ready")
        }
    } else {
        return errors.New("invalid")
    }
}

// ✅ After - Early returns
func ProcessItem(item Item) error {
    if !item.IsValid() {
        return errors.New("invalid")
    }

    if !item.IsReady() {
        return errors.New("not ready")
    }

    if !item.HasPermission() {
        return errors.New("no permission")
    }

    // Process
    return nil
}

Pattern 5: Switch Extraction (Long Switch)

Signal: Switch statement with complex cases

// ❌ Before - Long switch in one function
func HandleRequest(reqType string, data interface{}) error {
    switch reqType {
    case "create":
        // 20 lines of creation logic
    case "update":
        // 20 lines of update logic
    case "delete":
        // 15 lines of delete logic
    default:
        return errors.New("unknown type")
    }
    return nil
}

// ✅ After - Extracted handlers
func HandleRequest(reqType string, data interface{}) error {
    switch reqType {
    case "create":
        return handleCreate(data)
    case "update":
        return handleUpdate(data)
    case "delete":
        return handleDelete(data)
    default:
        return errors.New("unknown type")
    }
}

func handleCreate(data interface{}) error { /* ... */ }
func handleUpdate(data interface{}) error { /* ... */ }
func handleDelete(data interface{}) error { /* ... */ }

Refactoring Decision Tree

When linter fails, ask these questions (see reference.md for details):

1. Does this read like a story?

   • No → Extract functions for different abstraction levels

2. Can this be broken into smaller pieces?

   • By responsibility? → Extract functions
   • By task? → Extract functions
   • By category? → Extract functions

3. Does logic run on primitives?

   • Yes → Is this primitive obsession? → Extract type

4. Is function long due to switch statement?

   • Yes → Extract case handlers

5. Are there deeply nested if/else?

   • Yes → Use early returns or extract functions

After Refactoring

Verify

• Re-run task lintwithfix - Should pass
• Run tests - Should still pass
• Check coverage - Should maintain or improve
• Code more readable? - Get feedback if unsure

May Need

• New types created → Use @code-designing to validate design
• New functions added → Ensure tests cover them
• Major restructuring → Consider using @pre-commit-review

Output Format

🔧 REFACTORING COMPLETE

Linter Issues Resolved:
✅ user/service.go:45 - Cyclomatic complexity (15 → 8)
✅ user/handler.go:23 - Cognitive complexity (25 → 12)

Refactoring Applied:
1. Storifying: Extracted validateUser, saveUser, notifyUser
2. Extract Type: Created Port and ServiceAddress types
3. Early Returns: Reduced nesting in ProcessItem

Files Modified:
- user/service.go (+30, -45 lines)
- user/port.go (new file, +25 lines)
- user/address.go (new file, +35 lines)

Next Steps:
1. Re-run linter: task lintwithfix → Should pass
2. Run tests: go test ./... → Should pass
3. If new types created → Consider @code-designing review
4. Proceed to @pre-commit-review phase

Learning from Examples

For real-world refactoring case studies that show the complete thought process:

Example 1: Storifying Mixed Abstractions

• Transforms a 48-line fat function into lean orchestration + leaf type
• Shows how to extract IPConfig type for collection and validation logic
• Demonstrates achieving 100% unit test coverage without mocking

Example 2: Primitive Obsession with Multiple Types

• Transforms a 60-line function into a 7-line story by extracting 4 leaf types
• Shows the Type Alias Pattern for config-friendly types
• Demonstrates eliminating switch statement duplication
• Fixed misleading function name (validateCIDR → alignCIDRArgs)

See examples.md for complete case studies with thought process.

Integration with Other Skills

• @code-designing: When refactoring creates new types, validate design
• @testing: Ensure refactored code maintains test coverage
• @pre-commit-review: Final validation before commit

See reference.md for complete refactoring patterns and decision tree.

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