[jira] [Created] (ARROW-15471) [R] ExtensionType support in R

["Dewey Dunnington (Jira)" <ji...@apache.org>]

Dewey Dunnington created ARROW-15471:
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             Summary: [R] ExtensionType support in R
                 Key: ARROW-15471
                 URL: https://issues.apache.org/jira/browse/ARROW-15471
             Project: Apache Arrow
          Issue Type: Improvement
          Components: R
            Reporter: Dewey Dunnington


In Python there is support for extension types that consists of a registration step that defines functions to handle metadata serialization and deserialization. In R, any extension name or metadata at the top level is currently obliterated on import. To implement geometry reading and writing to Parquet, IPC, and/or Feather, we will need to at the very least have the extension name and metadata preserved (in R), and at best provide a registration step to customize the behaviour of the resulting Array/DataType.

Reprex for R:

{code:R}
# remotes::install_github("paleolimbot/narrow")
library(narrow)

carray <- as_narrow_array(1:5)

carray$schema$metadata[["ARROW:extension:name"]] <- "extension name!"
carray$schema$metadata[["ARROW:extension:metadata"]] <- "bananas"
carray$schema$metadata[["something else"]] <- "more bananas"

array <- from_narrow_array(carray, arrow::Array)
carray2 <- as_narrow_array(array)

carray2$schema$metadata[["ARROW:extension:name"]]
#> NULL
carray2$schema$metadata[["ARROW:extension:metadata"]]
#> NULL
carray2$schema$metadata[["something else"]]
#> NULL
{code}


There is some discussion of that as a solution to ARROW-14378, including an example of how pandas implements the 'interval' extension type (example contributed by [~jorisvandenbossche]).

For the Interval example, there are some different parts living in different places:

- The Arrow Extension Type definition for pandas' interval type: https://github.com/pandas-dev/pandas/blob/fc6b441ba527ca32b460ae4f4f5a6802335497f9/pandas/core/arrays/_arrow_utils.py#L88-L136
- The __from_arrow__ implementation (doing the conversion to arrow): https://github.com/pandas-dev/pandas/blob/fc6b441ba527ca32b460ae4f4f5a6802335497f9/pandas/core/arrays/interval.py#L1405-L1455
- The __from_arrow__ implementation (conversion arrow -> pandas): https://github.com/pandas-dev/pandas/blob/fc6b441ba527ca32b460ae4f4f5a6802335497f9/pandas/core/dtypes/dtypes.py#L1227-L1255



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