[GitHub] [arrow] westonpace edited a comment on pull request #12609: ARROW-15067: [C++] Add tracing spans to the scanner

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westonpace edited a comment on pull request #12609:
URL: https://github.com/apache/arrow/pull/12609#issuecomment-1072793993


   > Should I first try to create something outside of future.h altogether, that just wraps around Then()?
   
   Put the code in lower down, not higher up.  In the Future::AddCallback and Future::Then methods you are correct, everything is confusing because:
    * Templates
    * We aren't allowed to create `Result<void>` so we do all kinds of games to avoid it
   
   So, for example, it is complaining because `Wrapstruct` is not a valid callback.  A callback for an empty future should look like `std::function<void(const Status& st)>`.  A callback for a non-empty future should look like `std::function<void(const Result<T>& result)>`
   
   Instead, you can make the change at `arrow::ConcreteFutureImpl::AddCallback` in `future.cc`.  By that point everything has been type erased and callback is `internal::FnOnce<void(const FutureImpl& impl)>`.


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