It seemed to me that the whole example was too contrived. The garage could simply have the option to buy the oil and give it to the mechanic. func GetOil(o

It seemed to me that the whole example was too contrived. The garage could simply have the option to buy the oil and give it to the mechanic. func GetOil(oilType OilType) Oil { ... } func ChangeOil(c Car, oil Oil) error { ... }