golang-error-handling

Go Error Handling

Modern error handling patterns for Go following 2025-2026 best practices.

Basic Error Handling

Always check errors:

result, err := doSomething()
if err != nil {
	return fmt.Errorf("operation failed: %w", err)
}

Never ignore errors (unless explicitly documented):

// Bad
doSomething()

// Good - explicit discard when safe
_ = file.Close()

// Good - handle in defer with function
defer func() {
	if err := file.Close(); err != nil {
		log.Printf("failed to close file: %v", err)
	}
}()

Error Wrapping

Use %w to wrap errors:

func GetUser(id string) (*User, error) {
	user, err := db.Query(id)
	if err != nil {
		return nil, fmt.Errorf("failed to get user %s: %w", id, err)
	}
	return user, nil
}

Unwrap errors with errors.Is() and errors.As():

err := GetUser("123")
if errors.Is(err, sql.ErrNoRows) {
	// Handle not found
}

var validationErr *ValidationError
if errors.As(err, &validationErr) {
	// Handle validation error
	fmt.Printf("field: %s, message: %s", validationErr.Field, validationErr.Message)
}

Sentinel Errors

Define package-level error values:

package services

import "errors"

var (
	ErrNotFound      = errors.New("resource not found")
	ErrAlreadyExists = errors.New("resource already exists")
	ErrInvalidInput  = errors.New("invalid input")
	ErrConflict      = errors.New("resource conflict")
	ErrUnauthorized  = errors.New("unauthorized")
)

Check with errors.Is():

session, err := service.GetSession(ctx, id)
if errors.Is(err, services.ErrNotFound) {
	return http.StatusNotFound, "session not found"
}
if err != nil {
	return http.StatusInternalServerError, "internal error"
}

Custom Error Types

For errors needing additional context:

type ValidationError struct {
	Field   string
	Message string
}

func (e *ValidationError) Error() string {
	return fmt.Sprintf("validation error: %s %s", e.Field, e.Message)
}

func NewValidationError(field, message string) error {
	return &ValidationError{Field: field, Message: message}
}

// Usage
if req.SessionID == "" {
	return nil, NewValidationError("session_id", "required")
}