Implement vec_chop2()

[lionel-]

Add vec_chop2() which is the [[ equivalent of vec_chop(). It is still unexported for now because we probably don't want to rush the "2" API but it will be useful internally for implementing vec_map().

With atomic vectors it always transforms inner names into outer names:

vec_chop2(c(foo = 1, bar = 2))
#> $foo
#> [1] 1
#>
#> $bar
#> [1] 2

df <- data.frame(x = 1:2, row.names = c("foo", "bar"))
vec_chop2(df)
#> $foo
#>   x
#> 1 1
#>
#> $bar
#>   x
#> 1 2

mat <- matrix(1:4, 2, dimnames = list(c("foo", "bar"), c("x", "y")))
vec_chop2(mat)
#> $foo
#>      x y
#> [1,] 1 3
#>
#> $bar
#>      x y
#> [1,] 2 4

Since we now require lists to have list storage, it just unstructures them:

x <- list(x = 1, y = 2)
vec_chop2(new_vctr(x))
#> $x
#> [1] 1
#>
#> $y
#> [1] 2

Having this storage requirement really simplifies working with lists. We won't be able to support S4 lists like S4Vectors::List() without introducing something like list_proxy() and list_unproxy() though.

To make the implementation generic, vctrs_rcrd now supports assigning NULL names. This also fixes a bug as names<- would previously be applied to the fields. I still plan to add names support to rcrd vectors in the future but setting names to a character vector is an error for now.

There is a new helper class for unit tests which is a record list that caches the sizes of its elements in attributes.

[DavisVaughan]

This looks correct to me! Do we care about indices? Maybe not right now? They are less useful than in vec-chop, I think.

[DavisVaughan]

I think this can modify x in place if it isn’t referenced? Maybe capture the vec_names earlier to prevent issues if that is the case?

[lionel-]

oh yeah

[lionel-]

Right we can probably wait until there is a need for indices.

[lionel-]

Having this storage requirement really simplifies working with lists. We won't be able to support S4 lists like S4Vectors::List() without introducing something like list_proxy() and list_unproxy() though.

Ah there's a feature request about this: #1164.