Documentation

Convert a query that might return zero rows to a query that always returns at least one row.

To speak in more concrete terms, optional is most useful to write LEFT JOINs.

Filter out MaybeTables, returning only the tables that are not-null.

This operation can be used to "undo" the effect of optional, which operationally is like turning a LEFT JOIN back into a full JOIN. You can think of this as analogous to catMaybes.

Extend an optional query with another query. This is useful if you want to step through multiple LEFT JOINs.

Note that traverseMaybeTable takes a a -> Query b function, which means you also have the ability to "expand" one row into multiple rows. If the a -> Query b function returns no rows, then the resulting query will also have no rows. However, regardless of the given a -> Query b function, if the input is nothingTable, you will always get exactly one nothingTable back.