Support user functions in Sqlite3 driver

[bcc32]

Sqlite supports a notion of "application-defined SQL functions", namely, functions that can be used in SQL expressions but which are implemented by the user of the database driver (in this case, in OCaml). sqlite3-ocaml supports the necessary bindings already.

I understand that this probably wouldn't apply to the other database drivers, and it seems like quite a substantial task to make it work with the rest of the API, but I thought it might be reasonable to open this as a tracking issue. Please close if you disagree.

[paurkedal]

I think a way to support this and some other RDBMS specific features would be to make the underlying connection object available, e.g. using an extensible variant which can be accessed from a constructor defined by the driver module. Code which uses this would need to link against the driver (instead of relying on dynamic loading), but that's probably acceptable in this case.

[paurkedal]

Sorry for the long wait. The solution isn't ideal, but commit a04466f provides access to the connection handle for sqlite3. It requires linking against the caqti-driver-sqlite3 library. The test shows how it can be used:

let test_fun (module C : Caqti_blocking.CONNECTION) =
  (match C.driver_connection with
   | Some Caqti_driver_sqlite3.Driver_connection db ->
      Sqlite3.create_fun1 db "trim" begin function
       | TEXT s -> TEXT (String.trim s)
       | x -> x
      end;
      assert (C.find trim_req "  . " = Ok ".")
   | Some _ -> assert false
   | None -> assert false)

[bcc32]

Thanks so much! I finally got around to working on the project that needed this and it worked perfectly. I like the new infix Caqti_request interface too; it's very intuitive :)