[GitHub] [arrow] jonkeane commented on pull request #34744: GH-15248: [R] R to Arrow to R roundtrip adds a ptype column attribute to list columns

["jonkeane (via GitHub)" <gi...@apache.org>]

jonkeane commented on PR #34744:
URL: https://github.com/apache/arrow/pull/34744#issuecomment-1487418207

   Agreed with @paleolimbot our implementation of arrow lists <-> list columns was always a bit funny (and not entirely fully fleshed out). I remember one feature we do use that for is columns that contain data.frames and IIRC, there's some special casing of that in our own code.
   
   > I think the root cause is that our "arrow_list" vctrs class is half-baked (not necessarily that adding the .ptype or returning a vctrs_list_of is a problem.
   > 
   > (Alternatively, you could fix the vctrs implementation for arrow_list and friends but unless somebody pipes up in defense of it I think we should wipe all references to it and commit to vctrs_list_of)
   
   It would be great to briefly breakdown what these two would mean pros/cons that kind of thing — it doesn't have to be exhaustive, but I don't have a good list in my head right now of why one would be better than the other.
   
   Another thing I might be misremembering (so take it with a grain of salt + verify I'm not making this up): there is a slight mistmatch between what kind of heterogeneity is allowed in an R list versus and Arrow list. IIRC, (for variable sized lists) Arrow still requires types to be consistent across elements, but R does not. That should be totally ok with roundtripping arrow -> R -> arrow, though we might need to catch if an R list isn't compatible. 


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