tidy grouped data attributes

[romainfrancois]

Would it make sense to tidy attributes that we use internally for grouped data frame, we could e.g. have labels, indices and indices sizes in the same data frame, which would be convenient for manipulation because then it's tidy. (inspired by working on #341).

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
df <- data.frame(x = 1:2, y = factor(c(1, 1), levels = 1:2))
gdf <- group_by(df, x, y)

attributes(gdf)[ c("labels", "indices", "group_sizes")]
#> $labels
#>   x y
#> 1 1 1
#> 2 2 1
#> 
#> $indices
#> $indices[[1]]
#> [1] 0
#> 
#> $indices[[2]]
#> [1] 1
#> 
#> 
#> $group_sizes
#> [1] 1 1
as_tibble(
  mutate( attr(gdf, "labels"), 
    ..index.. = attr(gdf, "indices"), 
    ..size.. = attr(gdf, "group_sizes")
  )
)
#> # A tibble: 2 x 4
#>       x y     ..index.. ..size..
#>   <int> <fct> <list>       <int>
#> 1     1 1     <int [1]>        1
#> 2     2 1     <int [1]>        1

Created on 2018-04-09 by the reprex package (v0.2.0).

[krlmlr]

I think so. Added to the list of breaking changes which we can start working on right after the upcoming release.

[romainfrancois]

Maybe this is part of a bigger change on how we do groupings. Having something tidy could perhaps open the door to other kinds of groupings. Way back we discussed bootstrap groupings for example.

The way things are done now with grouped and rowwise is less than ideal

[krlmlr]

Seems related to #2311 then?