Unexpected summarise() interaction with sf objects

[cphaarmeyer]

Since {dplyr} 1.1.0 there is the (experimental) .by argument in some verbs as a group-by-alternative. I tripped over an unexpected interaction with summarise()and sf objects.

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
library(sf)
#> Linking to GEOS 3.9.3, GDAL 3.5.2, PROJ 8.2.1; sf_use_s2() is TRUE

nc <- read_sf(system.file("shape/nc.shp", package = "sf"))
nc$area_cl <- cut(nc$AREA, c(0, .1, .12, .15, .25))

nc |>
  group_by(area_cl) |>
  summarise(mean(AREA))
#> Simple feature collection with 4 features and 2 fields
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS:  NAD27
#> # A tibble: 4 × 3
#>   area_cl     `mean(AREA)`                                              geometry
#>   <fct>              <dbl>                                    <MULTIPOLYGON [°]>
#> 1 (0,0.1]           0.0760 (((-76.68874 36.29452, -76.72651 36.15682, -76.69016…
#> 2 (0.1,0.12]        0.112  (((-80.93127 35.61959, -81.00358 35.69706, -81.05478…
#> 3 (0.12,0.15]       0.134  (((-76.46926 34.69328, -76.2877 34.87701, -76.37468 …
#> 4 (0.15,0.25]       0.190  (((-80.76612 35.68204, -80.70597 35.85166, -80.68958…

# geometries are all the same
nc |>
  summarise(mean(AREA), .by = area_cl)
#> Simple feature collection with 4 features and 2 fields
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS:  NAD27
#> # A tibble: 4 × 3
#>   area_cl     `mean(AREA)`                                              geometry
#>   <fct>              <dbl>                                    <MULTIPOLYGON [°]>
#> 1 (0.1,0.12]        0.112  (((-75.97629 36.51793, -75.97728 36.47802, -75.9248 …
#> 2 (0,0.1]           0.0760 (((-75.97629 36.51793, -75.97728 36.47802, -75.9248 …
#> 3 (0.12,0.15]       0.134  (((-75.97629 36.51793, -75.97728 36.47802, -75.9248 …
#> 4 (0.15,0.25]       0.190  (((-75.97629 36.51793, -75.97728 36.47802, -75.9248 …

# this ist the expected result
nc |>
  summarise(mean(AREA), across(geometry, st_union), .by = area_cl)
#> Simple feature collection with 4 features and 2 fields
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS:  NAD27
#> # A tibble: 4 × 3
#>   area_cl     `mean(AREA)`                                              geometry
#>   <fct>              <dbl>                                    <MULTIPOLYGON [°]>
#> 1 (0.1,0.12]        0.112  (((-80.93127 35.61959, -81.00358 35.69706, -81.05478…
#> 2 (0,0.1]           0.0760 (((-76.68874 36.29452, -76.72651 36.15682, -76.69016…
#> 3 (0.12,0.15]       0.134  (((-76.46926 34.69328, -76.2877 34.87701, -76.37468 …
#> 4 (0.15,0.25]       0.190  (((-80.76612 35.68204, -80.70597 35.85166, -80.68958…

sf_extSoftVersion()
#>           GEOS           GDAL         proj.4 GDAL_with_GEOS     USE_PROJ_H 
#>        "3.9.3"        "3.5.2"        "8.2.1"         "true"         "true" 
#>           PROJ 
#>        "8.2.1"

^{Created on 2023-08-02 with reprex v2.0.2}

Session info

sessioninfo::session_info()
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#>  language (EN)
#>  collate  German_Germany.utf8
#>  ctype    German_Germany.utf8
#>  tz       Europe/Berlin
#>  date     2023-08-02
#>  pandoc   3.1.1 @ C:/Program Files/RStudio/resources/app/bin/quarto/bin/tools/ (via rmarkdown)
#> 
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