What are error types and error values in Go?

Question

Accepted Answer

Error types and values are two ways to **create errors** in Go.

```go
import (
    "errors"
    "fmt"
)

// 1. Error values (sentinel errors)
var (
    ErrNotFound     = errors.New("not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrForbidden    = errors.New("forbidden")
    ErrInternal     = errors.New("internal error")
)

// 2. Error types (with context)
type NotFoundError struct {
    Resource string
    ID       interface{}
}

func (e *NotFoundError) Error() string {
    return fmt.Sprintf("%s with ID %v not found", e.Resource, e.ID)
}

// 3. Wrapped errors (chain)
type AppError struct {
    Code    string
    Message string
    Err     error
}

func (e *AppError) Error() string {
    return fmt.Sprintf("[%s] %s", e.Code, e.Message)
}

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

// Usage
func GetUser(id int) (*User, error) {
    user, err := db.Find(id)
    if err != nil {
        if errors.Is(err, sql.ErrNoRows) {
            return nil, &NotFoundError{
                Resource: "User",
                ID:       id,
            }
        }
        return nil, &AppError{
            Code:    "DB_ERROR",
            Message: "database query failed",
            Err:     err,
        }
    }
    return user, nil
}

// Checking
user, err := GetUser(42)
if err != nil {
    var notFound *NotFoundError
    if errors.As(err, &notFound) {
        http.Error(w, notFound.Error(), 404)
    }

var appErr *AppError
    if errors.As(err, &appErr) {
        log.Printf("Code: %s, Message: %s", appErr.Code, appErr.Message)
    }
}
```

**Difference:**
- Error values: simple, static, for checking with errors.Is.
- Error types: contain context, for extraction with errors.As.

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