Use new_partial() when constructing a new partial type subclass;
and use is_partial() to test if an type is partial. All subclasses
need to provide a vec_type_finalise() method.
new_partial(..., class = character())is_partial(x)
vec_type_finalise(x)
Attributes of the partial type
Name of subclass.
As the name suggests, a partial type partially specifies a type, and
it must be combined with data to yield a full type. A useful example
of a partial type is partial_frame(), which makes it possible to
specify the type of just a few columns in a data frame. Use this constructor
if you're making your own partial type.