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

[GitBox <gi...@apache.org>]

paleolimbot commented on pull request #12467:
URL: https://github.com/apache/arrow/pull/12467#issuecomment-1075121124


   The key step that was missing for the roundtrip was `register_extension_type()`, which is needed so that Arrow C++ knows not to discard the extension metadata when it encounters the type! (see details).
   
   I should probably export `ExtensionArray` and use `ExtensionArray$create()` rather than `new_extension_array()` since it's more arrow-ish to do that. Maybe `ExtensionType$create()` instead of `new_extension_type()` is where extension type creation should go, too.
   
   Printing is a good point...definitely confusing in the case of an extension type!
   
   <details>
   
   ``` r
   library(arrow, warn.conflicts = FALSE)
   
   KoreanAge <- R6::R6Class(
     "KoreanAge", 
     inherit = ExtensionType,
     public = list(
       .array_as_vector = function(extension_array) {
         extension_array$storage()$as_vector() + 1
       }
     )
   )
   
   # constructor helpers
   korean_age <- function() {
     new_extension_type(
       int32(),
       "KoreanAge",
       charToRaw("Korean Age, but stored as the western age value"),
       type_class = KoreanAge
     )
   }
   
   korean_age_array <- function(age_korean) {
     new_extension_array(age_korean - 1, korean_age())
   }
   
   (arr <- korean_age_array(1:3))
   #> ExtensionArray
   #> <KoreanAge <Korean Age, but stored as the western age value>>
   #> [
   #>   0,
   #>   1,
   #>   2
   #> ]
   as.vector(arr)
   #> [1] 1 2 3
   
   # you need to register the type for Arrow C++ to keep the extension type
   # slash metadata when it encounters it at the C++ level (import from C
   # and reading files)
   register_extension_type(korean_age())
   
   tf <- tempfile()
   write_feather(arrow_table(col = arr), tf)
   
   tab <- read_feather(tf, as_data_frame = FALSE)
   
   type(tab$col)
   #> KoreanAge
   #> KoreanAge <Korean Age, but stored as the western age value>
   
   as.vector(tab$col)
   #> [1] 0 1 2
   ```
   
   </details>


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