Go 1.13 const error example

go13_const_err.go

pacakge x

const (
	ErrPermissoinDenied = constErr("permissoin denied")
	ErrInvalidParameters = constErr("invalid parameters")
)

type constErr string

func (err constErr) Is(target error) bool {
	s := target.Error()
	if s == string(err) || strings.HasPrefix(s, string(err)+": ")
}

func (err constErr) wrap(err error) error {
	return wrapErr{msg: string(err),  err: err}
}

type wrapErr struct {
	msg string
	err error
}

func (err wrapErr) Is(target error) bool {
	return constErr(err.msg).Is(target)
}