errors.go

errors.New("invalid parameter a")
errors.New("invalid parameter b")
errors.New("Record Not Found")

errors_revised.go

var ErrRecordNotFound = errors.New("Record Not Found")

type ErrParam struct {
  value string
}

func (e *ErrParam) Error() string {
    return "invalid paramater " + e.value
}

func NewErrParam(value string) ErrParam {
  return ErrParam{value: value}
}

NewErrParam("a")
NewErrParam("b")
ErrRecordNotFound