What is the difference between errors.Is and errors.As?

Question

Accepted Answer

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

func findUser(id string) error {
    return fmt.Errorf("findUser: %w", ErrNotFound) // wrapped
}

err := findUser("123")
if errors.Is(err, ErrNotFound) {
    // true — errors.Is unwraps the chain
    fmt.Println("user not found")
}

// Typed errors — use errors.As
type ValidationError struct {
    Field   string
    Message string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("%s: %s", e.Field, e.Message)
}

func validate(data Data) error {
    return fmt.Errorf("validate: %w", &ValidationError{
        Field:   "email",
        Message: "invalid format",
    })
}

err2 := validate(data)
var valErr *ValidationError
if errors.As(err2, &valErr) {
    // true — extracted the typed error
    fmt.Println("field:", valErr.Field)
    fmt.Println("message:", valErr.Message)
} errors.Is: comparing against sentinel errors like sql.ErrNoRows, io.EOF, context.DeadlineExceeded errors.As: extracting typed errors to access additional fields (HTTP status code, DB error code) Both unwrap the error chain — wrapping with %w is essential for them to work errors.Is uses == comparison by default; implement Is(error) bool on custom types for custom matching errors.As uses type assignability — it finds the first error in chain assignable to the target type

What is the difference between errors.Is and errors.As?

errors.Is checks if any error in the chain matches a specific sentinel error by value. errors.As checks if any error in the chain can be assigned to a specific type and extracts it.

What is the difference between errors.Is and errors.As?

errors.Is checks if any error in the chain matches a specific sentinel error by value. errors.As checks if any error in the chain can be assigned to a specific type and extracts it.

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