ydb/yql/essentials/tests/s-expressions/suites/Builtins/Fold1.yql

(
#comment
(let mr_source (DataSource 'yt 'plato))
(let x (Read! world mr_source (Key '('table (String 'Input))) '('key 'subkey 'value) '()))
(let world (Left! x))
(let table1 (Right! x))
(let table1map (FlatMap table1 (lambda '(item) (block '(
   (let value (Member item 'value))
   (let newValue (AsList value))
   (let newValue (Prepend (String 'prepended) newValue))
   (let newValue (Append newValue (String 'appended)))
   (let state (Fold1 newValue 
       (lambda '(item) '((Size item) (Uint64 '1))) 
       (lambda '(item state) '((+ (Nth state '0) (Size item)) (+ (Nth state '1) (Uint64 '1))))))
   (return (FlatMap state (lambda '(state) (block '(
      (let s (Struct))
      (let s (AddMember s 'key (Member item 'key)))
      (let s (AddMember s 'subkey (String  '.)))
      (let x (ToString (Nth state '0)))
      (let y (ToString (Nth state '1)))
      (let s (AddMember s 'value 
         (Concat (Concat value (String '" ==> ")) (Concat (Concat x (String '" ")) y))))
      (return (AsList s))
   )))))
)))))
(let mr_sink (DataSink 'yt (quote plato)))
(let world (Write! world mr_sink (Key '('table (String 'Output))) table1map '('('mode 'append))))
(let world (Commit! world mr_sink))
(return world)
)