jonkeane commented on pull request #8650:
URL: https://github.com/apache/arrow/pull/8650#issuecomment-788404473
Ok, I've looked at this more, and my first reprex was a bit _too_ minimal it
looks like. Here is another example/test we could/should add to exercise this
(I'm happy to push a commit including it to this branch if you would like):
```
test_that("sf-like list columns", {
df <- structure(list(col =
structure(list(structure(list(list(structure(1))), class = "inner")), class =
"outer")), class = "data.frame")
expect_array_roundtrip(df)
})
```
The fix that you made does fix an error on the `inner` "class", but I
believe that
https://github.com/apache/arrow/blob/fe1c774813e9ce7123f6bb02c43bca1664e8370b/r/src/r_to_arrow.cpp#L1014
is tripping on the `outer` "class".
This was extra-fun to debug (and explains why the sf example above worked)
because it looks like sf [registers some
vctrs](https://github.com/r-spatial/sf/blob/master/R/tidyverse-vctrs.R) methods
which mean that these will work so long as those have been registered (i.e.
whenever `sf` is used).
I suspect that it would be very infrequent to have someone want to
round-trip a parquet file including sf data without also having sf loaded (so
in practice the current state would be fine), attempting to debug what's going
on is super complicated (and any other non-standard vctrs that use that outer
level style classing would similarly fail).
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