`unnest_wider()` drops column if it contains empty lists

[RoelVerbelen]

Inconsistency in output format that came up when processing nested lists that might be empty:

library(tidyr)
df <- tibble(
  x = list(list(z = list(1)), list(z = list(2))), 
  y = 1:2
  )
# column z as expected
df %>% unnest_wider(x)
#> # A tibble: 2 x 2
#>   z              y
#>   <list>     <int>
#> 1 <list [1]>     1
#> 2 <list [1]>     2


df <- tibble(
  x = list(list(z = list()), list(z = list())), 
  y = 1:2
)
# column z not present
df %>% unnest_wider(x)
#> # A tibble: 2 x 1
#>       y
#>   <int>
#> 1     1
#> 2     2

^{Created on 2021-06-10 by the reprex package (v1.0.0)}

[DavisVaughan]

A similar example:

library(tidyr)

# Mix of empty types
col <- list(
  list(a = list()),
  list(a = integer())
)

df <- tibble(col = col)
df
#> # A tibble: 2 × 1
#>   col             
#>   <list>          
#> 1 <named list [1]>
#> 2 <named list [1]>

# Lost the `a` col
unnest_wider(df, col)
#> # A tibble: 0 × 0


# Mix of empty types and character vectors
col <- list(
  list(a = "x"),
  list(a = list()),
  list(a = integer()),
  list(a = "y")
)

df <- tibble(col = col)
df
#> # A tibble: 4 × 1
#>   col             
#>   <list>          
#> 1 <named list [1]>
#> 2 <named list [1]>
#> 3 <named list [1]>
#> 4 <named list [1]>

# WAT. How could those be combined with the character vectors?
unnest_wider(df, col, simplify = TRUE)
#> # A tibble: 4 × 1
#>   a    
#>   <chr>
#> 1 x    
#> 2 <NA> 
#> 3 <NA> 
#> 4 y

# They could be combined because they were replaced with `NULL`.
unnest_wider(df, col, simplify = FALSE)
#> # A tibble: 4 × 1
#>   a        
#>   <list>   
#> 1 <chr [1]>
#> 2 <NULL>   
#> 3 <NULL>   
#> 4 <chr [1]>

^{Created on 2021-11-03 by the reprex package (v2.0.1)}

This happens because of this compact() that removes size zero elements (as well as NULL elements), and I feel like that is wrong as it can result in some unnesting being allowed (as seen above)

tidyr/R/rectangle.R

Line 432 in 7d2a2cb

x <- purrr::compact(x)

[DavisVaughan]

This actually affects the vignette for rectangling, which relies on the current behavior. It can be summed up as follows:

library(tidyr)
library(repurrrsive)

chars <- tibble(char = got_chars)
head(chars)
#> # A tibble: 6 × 1
#>   char             
#>   <list>           
#> 1 <named list [18]>
#> 2 <named list [18]>
#> 3 <named list [18]>
#> 4 <named list [18]>
#> 5 <named list [18]>
#> 6 <named list [18]>

# Inside each row is an `aliases` element.
# It is always a character vector except in this one case:
chars$char[[18]]$aliases
#> [1] "Catelyn Tully"     "Lady Stoneheart"   "The Silent Sistet"
#> [4] "Mother Mercilesr"  "The Hangwomans"
chars$char[[19]]$aliases
#> list()
chars$char[[20]]$aliases
#> [1] "Ned"            "The Ned"        "The Quiet Wolf"

# Unnest, then look at that aliases column
chars2 <- chars %>% unnest_wider(char)

# Should be:
# <chr  [5]>
# <list [0]>
# <chr  [3]>
chars2[18:20, "aliases"]
#> # A tibble: 3 × 1
#>   aliases  
#>   <list>   
#> 1 <chr [5]>
#> 2 <NULL>   
#> 3 <chr [3]>

# Current implementation means this works, but I don't
# think it should
unnest_longer(chars2[18:20, "aliases"], aliases)
#> # A tibble: 9 × 1
#>   aliases          
#>   <chr>            
#> 1 Catelyn Tully    
#> 2 Lady Stoneheart  
#> 3 The Silent Sistet
#> 4 Mother Mercilesr 
#> 5 The Hangwomans   
#> 6 <NA>             
#> 7 Ned              
#> 8 The Ned          
#> 9 The Quiet Wolf

# This should have been identical to:
vctrs::vec_c(
  chars$char[[18]]$aliases,
  chars$char[[19]]$aliases,
  chars$char[[20]]$aliases
)
#> Error: Can't combine `..1` <character> and `..2` <list>.

^{Created on 2021-11-03 by the reprex package (v2.0.1)}

[mgirlich]

Note that, unfortunately, empty lists are a bit special because they easily appear when working with JSON:

json <- jsonlite::toJSON(
  list(
    list(x = 1:3),
    list(x = integer()),
    list(x = 4:5)
  )
)
json
#> [{"x":[1,2,3]},{"x":[]},{"x":[4,5]}]

jsonlite::fromJSON(json, simplifyDataFrame = FALSE)
#> [[1]]
#> [[1]]$x
#> [1] 1 2 3
#> 
#> 
#> [[2]]
#> [[2]]$x
#> list()
#> 
#> 
#> [[3]]
#> [[3]]$x
#> [1] 4 5
tibble::tibble(jsonlite::fromJSON(json, simplifyDataFrame = TRUE))
#> # A tibble: 3 × 1
#>   x        
#>   <list>   
#> 1 <int [3]>
#> 2 <int [0]>
#> 3 <int [2]>

^{Created on 2021-11-04 by the reprex package (v2.0.1)}

And I guess the rectangling functions are typically used when working with JSON.

So, it might make sense to allow (via an argument?) empty lists to be combined with other vectors.

[DavisVaughan]

Yea I was thinking the same thing! I'll likely add an argument for some form of that