Error with to_spatial_smooth when there is no pseudo-node

[agila5]

Describe the bug
I think that the function to_spatial_smooth() should return a more informative error message in case there is no pseudo-node with the correct degree.

Reproducible example

# packages
library(sf)
library(tidygraph)
library(sfnetworks)

# data
my_l <- st_sfc(
  st_linestring(rbind(c(-1, 0), c(0, 0), c(1, 0))), 
  st_linestring(rbind(c(0, -1), c(0, 0), c(0, 1)))
)

# sfn structure
my_sfn <- as_sfnetwork(my_l, directed = FALSE)

# convert to spatial subdivide
my_sfn <- convert(my_sfn, to_spatial_subdivision, .clean = TRUE)
#> Warning: to_spatial_subdivision assumes attributes are constant over geometries
par(mar = rep(0, 4))
plot(my_sfn)

# convert to spatial smooth
convert(my_sfn, to_spatial_smooth)
#> Error in st_sf(x, ..., agr = agr, sf_column_name = sf_column_name): no simple features geometry column present

^{Created on 2021-01-29 by the reprex package (v0.3.0)}

It doesn't make any sense to "smooth" that network, but I think the function should simply return the original input, maybe with a warning message. Moreover, the error message is quite cryptic.

I think that the easiest solution is to add a check after the following code:

sfnetworks/R/morphers.R

Lines 550 to 555 in dd4b919

	## ==========================
	if (directed) {
	pseudo = degree(x, mode = "in") == 1 & degree(x, mode = "out") == 1
	} else {
	pseudo = degree(x) == 2
	}

to test if any node has the correct degree.

Expected behavior
A more informative warning/error message or return the input data.

R Session Info

sessionInfo()
#> R version 3.6.3 (2020-02-29)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 19041)
#> 
#> Matrix products: default
#> 
#> locale:
#> [1] LC_COLLATE=Italian_Italy.1252  LC_CTYPE=Italian_Italy.1252   
#> [3] LC_MONETARY=Italian_Italy.1252 LC_NUMERIC=C                  
#> [5] LC_TIME=Italian_Italy.1252    
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] sfnetworks_0.4.1 tidygraph_1.2.0  sf_0.9-7        
#> 
#> loaded via a namespace (and not attached):
#>  [1] Rcpp_1.0.6         compiler_3.6.3     pillar_1.4.7       highr_0.8         
#>  [5] class_7.3-17       tools_3.6.3        digest_0.6.27      gtable_0.3.0      
#>  [9] evaluate_0.14      lifecycle_0.2.0    tibble_3.0.6       pkgconfig_2.0.3   
#> [13] rlang_0.4.10       igraph_1.2.6       DBI_1.1.1          yaml_2.2.1        
#> [17] xfun_0.19          e1071_1.7-4        dplyr_1.0.3        stringr_1.4.0     
#> [21] knitr_1.30         sfheaders_0.4.0    generics_0.1.0     vctrs_0.3.6       
#> [25] classInt_0.4-3     grid_3.6.3         tidyselect_1.1.0   glue_1.4.2        
#> [29] R6_2.5.0           rmarkdown_2.5      ggplot2_3.3.2      purrr_0.3.4       
#> [33] tidyr_1.1.2        magrittr_2.0.1     scales_1.1.1       htmltools_0.5.0   
#> [37] ellipsis_0.3.1     units_0.6-7        assertthat_0.2.1   colorspace_2.0-0  
#> [41] KernSmooth_2.23-18 stringi_1.5.3      munsell_0.5.0      lwgeom_0.2-5      
#> [45] crayon_1.3.4

^{Created on 2021-01-29 by the reprex package (v0.3.0)}

[luukvdmeer]

Interesting! I would even say there should be no error message at all. The smoother should just return the original network when there are no pseudo nodes.