I agree, if he's open to one he should like the other too. But I don't think they’re "part & parcel".

We already have user-defined objects in printf, by virtue of their string-ize overload. %s will basically tell any object to render itself in the default manner.

What I suppose he's talking about are formats like d for decimal and h for hex. Maybe define a way to specify rectangular or polar for complex numbers, for example.

—John