Defining and extracting custom error types with errors.As

package main
 
import (
	"errors"
	"fmt"
)
 
type ValidationError struct {
	Field   string
	Message string
}
 
func (e *ValidationError) Error() string {
	return fmt.Sprintf("invalid %s: %s", e.Field, e.Message)
}
 
func validateUsername(name string) error {
	if len(name) < 3 {
		return &ValidationError{Field: "Username", Message: "too short"}
	}
	return nil
}
 
func main() {
	err := validateUsername("go")
	if err != nil {
		var valErr *ValidationError
		if errors.As(err, &valErr) {
			fmt.Printf("Validation failed on field: %s\n", valErr.Field)
		}
	}
}