A type for a column extractor, parameterized by the type of the rows. Each 'row value represents an individual row in the table.

make ~header ?align f declares a new column with header. The alignment defaults to Left. f is the function that should take care and encapsulate the knowledge of how the contents for this column is extracted and created for a given row. It is not immediately called but rather will be called during rendering (and thus if f raises, these exceptions will happen during the rendering part).