Github user MLnick commented on the issue:

    https://github.com/apache/spark/pull/17090
  
    Isn't deciding on the output schema for these methods essentially the same 
as deciding on transform semantics in #12574 (apart from the issue of how, or 
if, to have transform generate the "ground truth" items)?
    
    So in a way I'm not certain this "circumvents" the discussion around 
transform semantics / fitting into cross-validation & pipelines but rather 
makes an implicit decision on it. If this is set here, it would be rather 
awkward to have any future `transform` based recommend all functionality with a 
different output schema.
    
    My default choice in #12574 was to match the form of the existing `mllib` 
methods. That is the same as here, and matches the format for the existing 
`RankingMetrics` in `mllib`. So from that perspective it is the "easiest" 
choice.
    
    It's not necessarily the "best" choice - I go into the other option in 
detail on #12574 and the related JIRA. Ideally it should match up with the 
expected input schema for a new `RankingEvaluator` (not strictly necessary but 
definitely preferable).
    
    My concern here is that we make a quick decision implicitly due to time 
constraints and are stuck with it down the line as it is exposed in the public 
API. 


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