ydb/yql/essentials/tests/s-expressions/suites/InMem/StreamInGroupByKey.yqls

(
#comment
(let config (DataSource 'config))

(let res_sink (DataSink 'result))
(let list (AsList '((Uint32 '1) (String 'a)) '((Uint32 '1) (String 'b)) '((Uint32 '2) (String 'c))))
(let keyExtractor (lambda '(x) (Nth x '0)))
(let handler (lambda '(key list) (Iterator (AsList key key))))
(let world (Write! world res_sink (Key) (GroupByKey list keyExtractor handler) '('('type))))
(let world (Commit! world res_sink))
(return world)
)