Go Error Handling

Go Error Handling

Master Go's error handling patterns including error wrapping, sentinel errors, custom error types, and the errors package for robust applications.

Basic Error Handling

Creating and returning errors:

package main

import (
    "errors"
    "fmt"
)

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

func main() {
    result, err := divide(10, 0)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Println("Result:", result)
}

Using fmt.Errorf:

func processFile(filename string) error {
    if filename == "" {
        return fmt.Errorf("filename cannot be empty")
    }
    // Process file...
    return nil
}

Error Wrapping

Wrapping errors with context (Go 1.13+):

import (
    "errors"
    "fmt"
    "os"
)

func readConfig(path string) error {
    _, err := os.Open(path)
    if err != nil {
        return fmt.Errorf("failed to read config: %w", err)
    }
    return nil
}

func main() {
    err := readConfig("config.json")
    if err != nil {
        fmt.Println(err)
        // Output: failed to read config: open config.json: no such file
    }
}

Unwrapping errors:

func handleError(err error) {
    // Unwrap one level
    unwrapped := errors.Unwrap(err)
    if unwrapped != nil {
        fmt.Println("Unwrapped:", unwrapped)
    }

    // Check if specific error is in chain
    if errors.Is(err, os.ErrNotExist) {
        fmt.Println("File does not exist")
    }
}

Sentinel Errors

Defining and using sentinel errors:

package main

import (
    "errors"
    "fmt"
)

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

func getUser(id int) (string, error) {
    if id < 0 {
        return "", ErrInvalidInput
    }
    if id == 0 {
        return "", ErrNotFound
    }
    return fmt.Sprintf("user-%d", id), nil
}

func main() {
    _, err := getUser(0)
    if errors.Is(err, ErrNotFound) {
        fmt.Println("User not found")
    }
}

Custom Error Types

Implementing error interface:

type ValidationError struct {
    Field   string
    Message string
}

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

func validateAge(age int) error {
    if age < 0 {
        return &ValidationError{
            Field:   "age",
            Message: "must be positive",
        }
    }
    if age > 150 {
        return &ValidationError{
            Field:   "age",
            Message: "must be less than 150",
        }
    }
    return nil
}

func main() {
    err := validateAge(-5)
    if err != nil {
        fmt.Println(err)
    }
}

Type assertions with errors.As:

func handleValidation(err error) {
    var validationErr *ValidationError
    if errors.As(err, &validationErr) {
        fmt.Printf("Field '%s' failed: %s\n",
            validationErr.Field,
            validationErr.Message,
        )
    }
}

Multi-Error Handling

Collecting multiple errors:

type MultiError struct {
    Errors []error
}

func (m *MultiError) Error() string {
    if len(m.Errors) == 0 {
        return "no errors"
    }
    if len(m.Errors) == 1 {
        return m.Errors[0].Error()
    }
    return fmt.Sprintf("%d errors occurred: %v", len(m.Errors), m.Errors)
}

func (m *MultiError) Add(err error) {
    if err != nil {
        m.Errors = append(m.Errors, err)
    }
}

func validateUser(name, email string, age int) error {
    errs := &MultiError{}

    if name == "" {
        errs.Add(errors.New("name is required"))
    }
    if email == "" {
        errs.Add(errors.New("email is required"))
    }
    if age < 0 {
        errs.Add(errors.New("age must be positive"))
    }

    if len(errs.Errors) > 0 {
        return errs
    }
    return nil
}

Panic and Recover

When to use panic:

// Panic for unrecoverable errors
func mustConnect(dsn string) *DB {
    db, err := connect(dsn)
    if err != nil {
        panic(fmt.Sprintf("failed to connect to database: %v", err))
    }
    return db
}

// Recover from panics
func safeExecute(fn func()) (err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("panic recovered: %v", r)
        }
    }()

    fn()
    return nil
}

Error Handling Patterns

Early return pattern:

func processRequest(id int) error {
    user, err := fetchUser(id)
    if err != nil {
        return fmt.Errorf("fetch user: %w", err)
    }

    if err := validateUser(user); err != nil {
        return fmt.Errorf("validate user: %w", err)
    }

    if err := saveUser(user); err != nil {
        return fmt.Errorf("save user: %w", err)
    }

    return nil
}

Error variable naming:

// Good: specific error names
errDB := connectDB()
if errDB != nil {
    return fmt.Errorf("db connection: %w", errDB)
}

errCache := connectCache()
if errCache != nil {
    return fmt.Errorf("cache connection: %w", errCache)
}

// Avoid: reusing 'err' everywhere makes debugging harder

pkg/errors Pattern (Legacy)

Using github.com/pkg/errors:

import (
    "github.com/pkg/errors"
)

func loadConfig() error {
    _, err := os.Open("config.json")
    if err != nil {
        return errors.Wrap(err, "failed to load config")
    }
    return nil
}

func init() {
    if err := loadConfig(); err != nil {
        // Print stack trace
        fmt.Printf("%+v\n", err)
    }
}

Error Logging

Structured error logging:

import (
    "log/slog"
)

func processOrder(orderID string) error {
    order, err := fetchOrder(orderID)
    if err != nil {
        slog.Error("failed to fetch order",
            "orderID", orderID,
            "error", err,
        )
        return fmt.Errorf("fetch order %s: %w", orderID, err)
    }

    if err := validateOrder(order); err != nil {
        slog.Warn("order validation failed",
            "orderID", orderID,
            "error", err,
        )
        return fmt.Errorf("validate order: %w", err)
    }

    return nil
}

HTTP Error Handling

Handling HTTP errors:

import (
    "encoding/json"
    "net/http"
)

type APIError struct {
    Code    int    `json:"code"`
    Message string `json:"message"`
}

func (e *APIError) Error() string {
    return e.Message
}

func writeError(w http.ResponseWriter, err error) {
    var apiErr *APIError
    if errors.As(err, &apiErr) {
        w.WriteHeader(apiErr.Code)
        json.NewEncoder(w).Encode(apiErr)
        return
    }

    // Default error
    w.WriteHeader(http.StatusInternalServerError)
    json.NewEncoder(w).Encode(APIError{
        Code:    http.StatusInternalServerError,
        Message: "Internal server error",
    })
}

func handler(w http.ResponseWriter, r *http.Request) {
    err := processRequest(r)
    if err != nil {
        writeError(w, err)
        return
    }

    w.WriteHeader(http.StatusOK)
}

Error Context

Adding context to errors:

type ContextError struct {
    Op      string // Operation
    Path    string // File path, URL, etc.
    Err     error  // Underlying error
}

func (e *ContextError) Error() string {
    return fmt.Sprintf("%s %s: %v", e.Op, e.Path, e.Err)
}

func (e *ContextError) Unwrap() error {
    return e.Err
}

func readFile(path string) error {
    _, err := os.ReadFile(path)
    if err != nil {
        return &ContextError{
            Op:   "read",
            Path: path,
            Err:  err,
        }
    }
    return nil
}

Testing Error Cases

Testing error conditions:

package main

import (
    "errors"
    "testing"
)

func TestDivideByZero(t *testing.T) {
    _, err := divide(10, 0)
    if err == nil {
        t.Fatal("expected error, got nil")
    }

    expected := "division by zero"
    if err.Error() != expected {
        t.Errorf("expected %q, got %q", expected, err.Error())
    }
}

func TestErrorWrapping(t *testing.T) {
    err := readConfig("missing.json")
    if err == nil {
        t.Fatal("expected error")
    }

    if !errors.Is(err, os.ErrNotExist) {
        t.Error("expected wrapped ErrNotExist")
    }
}

func TestCustomError(t *testing.T) {
    err := validateAge(-1)

    var validationErr *ValidationError
    if !errors.As(err, &validationErr) {
        t.Fatal("expected ValidationError")
    }

    if validationErr.Field != "age" {
        t.Errorf("expected field 'age', got %q", validationErr.Field)
    }
}

When to Use This Skill

Use go-error-handling when you need to:

• Handle errors in Go applications properly
• Add context to errors without losing information
• Define domain-specific error types
• Check for specific error conditions
• Wrap errors with additional context
• Log errors with appropriate detail
• Return errors from HTTP handlers
• Test error conditions thoroughly
• Build error-resilient systems
• Implement retry logic based on error types

Best Practices

• Always check errors, never ignore them
• Return errors instead of logging and continuing
• Use fmt.Errorf with %w to wrap errors
• Use errors.Is for comparing sentinel errors
• Use errors.As for type assertions
• Provide context in error messages
• Use custom error types for domain errors
• Don't panic in libraries, return errors
• Log errors at appropriate levels
• Test error paths as thoroughly as happy paths

Common Pitfalls

• Ignoring errors with _ assignment
• Not wrapping errors (losing context)
• Using == for error comparison
• Panicking instead of returning errors
• Not handling all error cases
• Creating too many custom error types
• Poorly formatted error messages
• Not testing error conditions
• Swallowing errors in goroutines
• Not providing enough context in errors

Resources

• Go Blog - Error Handling
• Go Blog - Working with Errors
• Effective Go - Errors
• errors Package
• Error Handling in Go