3 simple ways to create an error

String-based errors

The standard library offers two out-of-the-box options.

// simple string-based error err1 := errors.New("math: square root of negative number") // with formatting err2 := fmt.Errorf("math: square root of negative number %g", x)

Custom errors with data

To define a custom error type, you must satisfy the predeclared error interface.

type error interface {	Error() string }

Here are two examples.

type SyntaxError struct {	Line int	Col int } func (e *SyntaxError) Error() string {	return fmt.Sprintf("%d:%d: syntax error", e.Line, e.Col) }

type InternalError struct {	Path string } func (e *InternalError) Error() string {	return fmt.Sprintf("parse %v: internal error", e.Path) }

If Foo is a function that can return a SyntaxError or an InternalError, you may handle the two cases like this.

if err := Foo(); err != nil {	switch e := err.(type) {	case *SyntaxError: // Do something interesting with e.Line and e.Col.	case *InternalError: // Abort and file an issue.	default:	log.Println(e)	} }

More code examples

Go blueprints: code for com­mon tasks is a collection of handy code examples.