Basic DB mapping

[Data type derivation being a pivotal point of HRR, explain more in detail
what HRR does at compile time and what the result looks like]

Full example

[Take a table like e.g. users and explain all the stuff HRR generates via Template Haskell:
the record data type, selectors, projections, and a basic relation that is equivalent to SELECT * FROM]


[Explain HRRs projections (:: Pi a b) and what they’re useful for]


[How to tell HRR to use custom type mappings, like mappings to different
Haskell string/text or numeric types]