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.

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