Unwrap Go errors from the go 2 errors proposal as well as github.com/pkg/errors

errors.go

package foo

// go2 errors
type wrapper interface {
	Unwrap() error
}

// pkg/errors
type causer interface {
	Cause() error
}

func unwrap(err error) error {
	switch e := err.(type) {
	case wrapper:
		return e.Unwrap()
	case causer:
		return e.Cause()
	}
	return err
}

Last return in unwrap can be under default statement inside the switch :)

func unwrap(err error) error {
	switch e := err.(type) {
	case wrapper:
		return e.Unwrap()
	case causer:
		return e.Cause()
	default:
		return err
	}
}