Character string uniquely identifying 'list', 'data.frame',
or 'vector'. If 'list', then each row of the data frame is output as a
list. If 'data.frame', then each row of the data frame is output as a
1-row data frame. If 'vector', then each row of the data frame is output
as a named vector. However, for 'vector,' each column of the data set must
be the same type. Defaults to 'list'.