go

Go — Advanced Patterns

Language-level patterns for writing correct, idiomatic, performant Go.

When to Activate

• Designing error types, wrapping, and sentinel errors
• Structuring goroutines, channels, and sync primitives correctly
• Propagating context.Context for cancellation and deadlines
• Composing interfaces and embedding types
• Using generics (any, constraints, type parameters) appropriately
• Writing table-driven tests, benchmarks, or fuzz targets
• Applying functional options, builder patterns, or the options struct pattern
• Structuring packages, modules, and internal vs exported APIs

---

Error Handling

Wrapping and unwrapping

import "errors"

// Wrap to add context — preserves the original for errors.Is/As
if err != nil {
    return fmt.Errorf("fetch user %d: %w", id, err)
}

// Sentinel errors — compare with errors.Is (not ==)
var ErrNotFound = errors.New("not found")
var ErrUnauthorized = errors.New("unauthorized")

if errors.Is(err, ErrNotFound) {
    // handle not found
}

// Custom error type — use errors.As to extract
type ValidationError struct {
    Field   string
    Message string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation error on %s: %s", e.Field, e.Message)
}

var ve *ValidationError
if errors.As(err, &ve) {
    log.Printf("invalid field: %s", ve.Field)
}

Multiple return errors

// BAD: ignoring errors
data, _ := os.ReadFile("config.json")

// GOOD: always handle
data, err := os.ReadFile("config.json")
if err != nil {
    return fmt.Errorf("read config: %w", err)
}

// Return early, keep the happy path unindented
func process(id string) (*Result, error) {
    user, err := db.GetUser(id)
    if err != nil {
        return nil, fmt.Errorf("get user: %w", err)
    }
    orders, err := db.GetOrders(id)
    if err != nil {
        return nil, fmt.Errorf("get orders: %w", err)
    }
    return &Result{User: user, Orders: orders}, nil
}

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Goroutines and Channels