Error Handling

Implement idiomatic Go error handling patterns.

Quick Start

Basic error handling:

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

Sentinel error:

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

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

Instructions

Step 1: Return Errors

Basic error return:

func ReadFile(path string) ([]byte, error) {
    data, err := os.ReadFile(path)
    if err != nil {
        return nil, fmt.Errorf("read file %s: %w", path, err)
    }
    return data, nil
}

Multiple return values:

func Divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil
}

Step 2: Wrap Errors with Context

Using fmt.Errorf with %w:

func ProcessFile(path string) error {
    data, err := ReadFile(path)
    if err != nil {
        return fmt.Errorf("process file: %w", err)
    }
    
    if err := Validate(data); err != nil {
        return fmt.Errorf("validate data: %w", err)
    }
    
    return nil
}

Error chain:

// Original error
err := os.Open("file.txt")

// Wrapped once
err = fmt.Errorf("open config: %w", err)

// Wrapped again
err = fmt.Errorf("initialize app: %w", err)

// Unwrap to check original
if errors.Is(err, os.ErrNotExist) {
    // Handle file not found
}

Step 3: Use Sentinel Errors

Define sentinel errors:

var (
    ErrNotFound     = errors.New("not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrInvalidInput = errors.New("invalid input")
)

func GetUser(id string) (*User, error) {
    user, ok := cache[id]
    if !ok {
        return nil, ErrNotFound
    }
    return user, nil
}

Check with errors.Is:

user, err := GetUser("123")
if errors.Is(err, ErrNotFound) {
    // Handle not found case
    return nil
}
if err != nil {
    // Handle other errors
    return err
}

Step 4: Create Custom Error Types

Error type with fields:

type ValidationError struct {
    Field string
    Value interface{}
    Msg   string
}

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

func Validate(user *User) error {
    if user.Email == "" {
        return &ValidationError{
            Field: "email",
            Value: user.Email,
            Msg:   "email is required",
        }
    }
    return nil
}

Check with errors.As:

if err := Validate(user); err != nil {
    var valErr *ValidationError
    if errors.As(err, &valErr) {
        fmt.Printf("Field %s failed: %s\n", valErr.Field, valErr.Msg)
    }
    return err
}

Step 5: Handle Errors Appropriately

Check immediately:

// Good
result, err := doSomething()
if err != nil {
    return err
}

// Bad - don't defer error checking
result, err := doSomething()
// ... more code ...
if err != nil {
    return err
}

Don't ignore errors:

// Bad
doSomething()

// Good
if err := doSomething(); err != nil {
    log.Printf("warning: %v", err)
}

// Or explicitly ignore
_ = doSomething()

Common Patterns

Error Wrapping Chain

func LoadConfig() (*Config, error) {
    data, err := readConfigFile()
    if err != nil {
        return nil, fmt.Errorf("load config: %w", err)
    }
    
    config, err := parseConfig(data)
    if err != nil {
        return nil, fmt.Errorf("load config: %w", err)
    }
    
    return config, nil
}

Multiple Error Returns

func ProcessBatch(items []Item) ([]Result, error) {
    results := make([]Result, 0, len(items))
    
    for _, item := range items {
        result, err := process(item)
        if err != nil {
            return nil, fmt.Errorf("process item %s: %w", item.ID, err)
        }
        results = append(results, result)
    }
    
    return results, nil
}

Collecting Multiple Errors

type MultiError []error

func (m MultiError) Error() string {
    var msgs []string
    for _, err := range m {
        msgs = append(msgs, err.Error())
    }
    return strings.Join(msgs, "; ")
}

func ValidateAll(users []*User) error {
    var errs MultiError
    
    for _, user := range users {
        if err := Validate(user); err != nil {
            errs = append(errs, err)
        }
    }
    
    if len(errs) > 0 {
        return errs
    }
    return nil
}

Panic and Recover

Use panic for programmer errors:

func MustCompile(pattern string) *regexp.Regexp {
    re, err := regexp.Compile(pattern)
    if err != nil {
        panic(err)  // Invalid pattern is programmer error
    }
    return re
}

Recover from panics:

func SafeExecute(fn func()) (err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("panic: %v", r)
        }
    }()
    
    fn()
    return nil
}

Error Context

type contextError struct {
    op  string
    err error
}

func (e *contextError) Error() string {
    return fmt.Sprintf("%s: %v", e.op, e.err)
}

func (e *contextError) Unwrap() error {
    return e.err
}

func withContext(op string, err error) error {
    if err == nil {
        return nil
    }
    return &contextError{op: op, err: err}
}

Error Handling in HTTP

func handler(w http.ResponseWriter, r *http.Request) {
    user, err := getUser(r.Context(), r.URL.Query().Get("id"))
    if errors.Is(err, ErrNotFound) {
        http.Error(w, "User not found", http.StatusNotFound)
        return
    }
    if errors.Is(err, ErrUnauthorized) {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    if err != nil {
        log.Printf("get user: %v", err)
        http.Error(w, "Internal error", http.StatusInternalServerError)
        return
    }
    
    json.NewEncoder(w).Encode(user)
}

Error Handling in Goroutines

func processAsync(items []Item) error {
    errCh := make(chan error, len(items))
    
    for _, item := range items {
        go func(it Item) {
            errCh <- process(it)
        }(item)
    }
    
    // Collect errors
    for range items {
        if err := <-errCh; err != nil {
            return fmt.Errorf("process failed: %w", err)
        }
    }
    
    return nil
}

Best Practices

1. Return errors, don't panic: Except for programmer errors
2. Check errors immediately: Don't defer checking
3. Wrap errors with context: Use fmt.Errorf with %w
4. Use sentinel errors: For expected error cases
5. Use custom types: For errors needing additional data
6. errors.Is for sentinel: Check wrapped sentinel errors
7. errors.As for types: Extract custom error types
8. Don't ignore errors: Handle or explicitly ignore with _
9. Error messages lowercase: Start with lowercase, no punctuation
10. Add context: Include operation and relevant data

Anti-Patterns

Don't:

• Ignore errors silently
• Use panic for normal errors
• Return error strings (use error types)
• Check error strings (use errors.Is/As)
• Create errors with fmt.Sprintf (use fmt.Errorf)
• Wrap errors without %w (loses error chain)

Troubleshooting

Error not matching with errors.Is:

• Ensure using %w when wrapping
• Check sentinel error is same instance

Can't extract error type:

• Use errors.As, not type assertion
• Ensure error type is pointer

Lost error context:

• Wrap errors at each level
• Include operation name in wrap