// Wrap with context
if err != nil {
    return fmt.Errorf("processing user %s: %w", userID, err)
}

// Sentinel errors
var ErrNotFound = errors.New("not found")

// Custom error type
type ValidationError struct {
    Field   string
    Message string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation failed for %s: %s", e.Field, e.Message)
}

// Go 1.20+ multiple errors
err := errors.Join(err1, err2, err3)

cmd/myapp/main.go    # Minimal - call into internal packages
internal/            # Private, cannot be imported externally
pkg/                 # Public library (optional)
testdata/            # Test fixtures (ignored by go build)

// Table-driven tests
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        expected int
    }{
        {"positive", 1, 2, 3},
        {"zero", 0, 0, 0},
    }
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            if got := Add(tt.a, tt.b); got != tt.expected {
                t.Errorf("Add(%d, %d) = %d, want %d", tt.a, tt.b, got, tt.expected)
            }
        })
    }
}

// Test helpers
func setupTestDB(t *testing.T) *DB {
    t.Helper()
    db := NewDB()
    t.Cleanup(func() { db.Close() })
    return db
}

// Context for cancellation
ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
defer cancel()

// Channel select
select {
case msg := <-ch1:
    handle(msg)
case <-ctx.Done():
    return ctx.Err()
}

// WaitGroup
var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func(item Item) {
        defer wg.Done()
        process(item)
    }(item)
}
wg.Wait()