How does error wrapping work in Go 1.13+ with errors.Is and errors.As?

Go 1.13 introduced a standardised error wrapping API. Errors can be wrapped using fmt.Errorf("... %w", err) to create a chain, and errors.Is / errors.As traverse that chain to find specific errors or extract their values.

import "errors"

// Sentinel errors — comparable with ==  or errors.Is
var ErrNotFound = errors.New("not found")
var ErrPermission = errors.New("permission denied")

// Wrap — %w creates an error chain
func openFile(path string) error {
    if err := os.Open(path); err != nil {
        return fmt.Errorf("openFile %s: %w", path, err) // wrap with context
    }
    return nil
}

// errors.Is — checks if any error in the chain equals the target
err := openFile("/nonexistent")
if errors.Is(err, os.ErrNotExist) {
    fmt.Println("file does not exist")
}

// Custom error type
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 validate(name string) error {
    if name == "" {
        return fmt.Errorf("user creation: %w", &ValidationError{"name", "required"})
    }
    return nil
}

// errors.As — extracts a specific type from the chain
err = validate("")
var ve *ValidationError
if errors.As(err, &ve) {
    fmt.Printf("Field: %s, Message: %s\n", ve.Field, ve.Message)
}

// errors.Unwrap — one level only
inner := errors.Unwrap(err) // returns the wrapped *ValidationError

// Multiple wrapping (Go 1.20+) — join multiple errors
err1, err2 := errors.New("e1"), errors.New("e2")
joined := errors.Join(err1, err2)
errors.Is(joined, err1) // true

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