Should vec_proxy_compare() have a fallback?

[hadley]

It would be nice to be able to sort data frames containing list columns. Should vec_proxy_compare() return seq_along(x)? Or should it return NULL, and then failures would be communicated downstream?

[hadley]

This is needed for (e.g.) tidyr::crossing() and nesting() because (for historical reasons) these sort all columns.

[DavisVaughan]

For sorting list columns we have changed usage vec_proxy_compare(relax = TRUE) to instead use vec_proxy_order(). This "orders" list column elements by first appearance, which seemed reasonable.

[lionel-]

@DavisVaughan We can now close this I think?

[DavisVaughan]

Yea, to close the loop:

We have vec_proxy_order.list() that allows you to sort lists by first appearance of the elements. We still error on vec_proxy_compare.list(), because comparison is not defined.

vctrs:::vec_proxy_order.list
#> function(x, ...) {
#>   # Order lists by first appearance.
#>   # This allows list elements to be grouped in `vec_order()`.
#>   # Have to separately ensure missing values are propagated.
#>   out <- vec_duplicate_id(x)
#>   na <- vec_equal_na(x)
#>   out <- vec_assign(out, na, NA_integer_)
#>   out
#> }
#> <bytecode: 0x7fbcc78e38e8>
#> <environment: namespace:vctrs>

vctrs:::vec_proxy_compare.list
#> function(x, ...) {
#>   stop_unsupported(x, "vec_proxy_compare")
#> }
#> <bytecode: 0x7fbcc7a575c0>
#> <environment: namespace:vctrs>

^{Created on 2022-09-12 with reprex v2.0.2}