Fair point about parametricity. A language could in the macro expansion do the equivalent of a scala implicit lookup for a sorting function for the type and ret

Fair point about parametricity. A language could in the macro expansion do the equivalent of a scala implicit lookup for a sorting function for the type and return an error at macro expansion time if it can't find one. That avoids the doing the right thing requires discipline problem but I agree it is still less clear from the type signature alone what the type requirements are.