[GitHub] [arrow] jorisvandenbossche edited a comment on pull request #9164: ARROW-10663: [C++] Fix is_in and index_in behaviour

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jorisvandenbossche edited a comment on pull request #9164:
URL: https://github.com/apache/arrow/pull/9164#issuecomment-758671039


   Thanks for the PR! 
   One additional thing I am wondering, while testing it out, would we ever want a behaviour where a `null` in the input gives a `null` in the output? Right now it's only possible to get `false` (if there is no null in the value_set, or if skip_nulls=True) or `true` (if there is a null in the value_set and skip_nulls=False).
   
   So something like `isin([1, 2, null], value_set=[1, 3]) -> [true, false, null]`
   
   If you see "isin"  as a shortcut to write multiple equality comparisons (`isin(input, value_set=[1, 3, ...]` -> `(input == 1) | (input == 3) | ...`), then you would get such behaviour. 
   But so it's a bit the question whether for "isin" we use "equality" semantics or "identity/lookup" semantics for nulls (and given it's now called "SetLookup" in the function names, we clearly go for the second, but I am not fully sure which of the two are most useful / expected in practice).


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