[GitHub] [arrow] jorisvandenbossche commented on pull request #12467: ARROW-15471: [R] ExtensionType support in R

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jorisvandenbossche commented on pull request #12467:
URL: https://github.com/apache/arrow/pull/12467#issuecomment-1075489604


   > I think so! This example is probably better
   
   That example indeed nicely shows that it is possible. 
   I suppose one thing that confused me is that `new_extension_type(..)` rather creates a new *instance* of a given type, and not a new type itself (and thus this allows you to create instances of a given extension type with different metadata).
   
   > It isn't all that straightforward to do (the way I've implemented it in R) and I'm not sure I like how it's implemented (but I'm also not sure how to make it better).
   
   I think the fact that you already determine the serialized metadata upfront in R is fine / nice (the fact that in Python this is C++ calling back into python is kind of a complication, as the metadata could be known at the point when instantiating the python extension type instance).  
   One thing I was wondering: the reason you can't define a `.Serialize` method on the QuantizedType that can hold this logic to create the serialized metadata (instead of doing that inside `new_extension_type( ..., extension_metadata=... )`), is that this class instance is only created after the underlying C++ extension type, for which you already need the serialized metadata? It's not possible to first initialize the QuantizedType object, and let that call Serialize to then create the underlying C++ object? (disclaimer: I know nothing about R classes :))
   
    


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