westonpace commented on a change in pull request #9945:
URL: https://github.com/apache/arrow/pull/9945#discussion_r609965523
##########
File path: cpp/src/arrow/util/async_generator.h
##########
@@ -1063,6 +1063,86 @@ AsyncGenerator<T>
MakeConcatenatedGenerator(AsyncGenerator<AsyncGenerator<T>> so
return MergedGenerator<T>(std::move(source), 1);
}
+template <typename T>
+struct Enumerated {
+ util::optional<T> value;
+ int index;
+ bool last;
Review comment:
Yes, it has to return one end token. If I return the end token for each
child stream I'll end up with 3. In other words, I want...
```
A1
B1
A2 (last-in-stream)
C1
C2 (last-in-stream)
B2 (last-in-stream)
END
```
...and not...
```
A1
B1
A2
END
C1
C2
END
B2
END
```
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