> That is SO disturbing... They don't even check whether the parameter implements another interface, they do the check directly on the method by callin it... Th

> That is SO disturbing... They don't even check whether the parameter implements another interface, they do the check directly on the method by callin it... That's so ugly... That's incorrect. The code you see at the top: if rt, ok := dst.(ReaderFrom); ok { return rt.ReadFrom(src) } means: "If dst implements ReaderFrom, then call ReadFrom on this value of that type." You can't arbitrarily call methods that may or may not exist. You can check if a value implements an interface by doing "newVal, ok := val.(suspectedInterface)", and checking if ok is true, or by doing "newVal := val.(suspectedInterface)" and living with a runtime panic if it doesn't implement the interface, then call a method in that interface on the new value newVal, which will be of type suspectedInterface. (If val doesn't implement suspectedInterface--i.e. ok is false--then newVal will be the zero value of suspectedInterface.)