Low priority

General type families for improving instance resolution (product-profunctors). (Currently there are three places in Opaleye where instance resolution needs to happen: runQuery, runInsertReturning and leftJoin. As it happens it seems difficult to make any of these benefit from a general type family approach.)

Good starter projects for someone wanting to contribute to Opaleye

Very easy

There may be some missing operators that just need to be written down
RIGHT JOIN, FULL OUTER JOIN
Set operations
- EXCEPT
- EXCEPT ALL
- UNION
- INTERSECT
- INTERSECT ALL
INSERT, UPDATE, DELETE RETURNING
Improve the testing "framework" perhaps by upgrading it to Tasty

Require a bit of work

Make the code generation neater
Make VALUES work with more, type checked, value types
Product-valued case statements and (.==)
Make the test database parameters more easily configurable
Randomised testing in a QuickCheck style
distinct, union and aggregate can be made to work with QueryArr rather than just Query if we use LATERAL JOIN