(
(let config (DataSource 'config))

(let list (AsList (String 'a) (String 'b) (String 'c)))
(let keyExtractor (lambda '(x) (String '"@")))
(let handler (lambda '(key list) (AsList (Uint32 '42))))
(let data (GroupByKey list keyExtractor handler))
(let res_sink (DataSink 'result))
(let world (Write! world res_sink
    (Key)
    data '('('type))))

(let world (Commit! world res_sink))
(return world)
)