Implement list_sizes()

[romainfrancois]

vec_sizes() is to vec_size() what lengths() is to length():

library(vctrs)

data <- list(iris, mtcars, 1:10)
vec_sizes(data)
#> [1] 150  32  10

The 😈 plan is to use this in a vctrs based implementation of things like dplyr::summarise() and dplyr::mutate() e.g. evaluate the expression in each group and then assert that the sizes are good (all equal to 1 if summaries, equal to the group sizes on a mutate).

The PR is wip, if we want that I'll add unit tests, etc ...

[romainfrancois]

Alternatively or additionally, it would be nice to have a C api for vec_size() that we can use from internals of e.g. dplyr.

[DavisVaughan]

Not sure if it matters, but it looks like this would also work on a data frame and return the size of each column (all the same size)?

[lionel-]

I think it'd be better to export vec_size() as a C callable, if needed.

Not sure if it matters, but it looks like this would also work on a data frame and return the size of each column (all the same size)?

That doesn't seem appropriate. In general this function seems ill-suited for vctrs at the moment because it's about [[.

[romainfrancois]

Sure. I can live with map_int(vec_size) for now, having vec_size() C callable means I can implement something that tests is all the sizes are 1 instead of getting all the sizes and then do a any(sizes != 1)

[lionel-]

Let's revisit once we have figured out vec_slice2() and the tbl_ API.

[lionel-]

From a discussion with @DavisVaughan, we could have list_sizes() that fails with atomic vectors (including dfs).

[DavisVaughan]

@romainfrancois I'll take over this PR if that's alright with you

[romainfrancois]

@DavisVaughan sure. dplyr still has a dplyr_vec_sizes but is unused at this point.

[DavisVaughan]

Pretty nice improvement

library(purrr)
library(vctrs)

x <- rep_len(list(1:5 + 0L), 100)

bench::mark(
  map_int(x, vec_size),
  list_sizes(x)
)
#> # A tibble: 2 x 6
#>   expression                min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>           <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 map_int(x, vec_size)  63.34µs  77.62µs    12802.    68.6KB     91.8
#> 2 list_sizes(x)          2.81µs   3.08µs   287798.     4.3KB      0

list_sizes(mtcars)
#> Error: `x` must be a list.

^{Created on 2020-03-18 by the reprex package (v0.3.0)}

[lionel-]

Minor: Do we need a registered alias since they have the same signature?

[DavisVaughan]

I tend to try and always keep a separate function prefixed with vctrs_ that gets exported, whether it is strictly required or not