When exporting a matrix-valued attribute, it is treated as numeric scalar

[szhorvat]

What happens, and what did you expect instead?

When exporting a matrix-valued attribute, it is treated as numeric scalar. The first element is exported, the rest are discarded.

CC @ntamas as this concerns the attribute interface as well, however, I believe this is in the end about how the R interface communicates the attribute type to the C core. Serializing into a string would perhaps be better.

To reproduce

library(igraphdata)
library(igraph)
data('kite')
write_graph(kite, 'kite.gml', 'gml')

Excerpts from the output:

graph
[
  directed 0
  name "Krackhardt's kite"
  layout 1

System information

R version 4.5.0 (2025-04-11)
Platform: aarch64-apple-darwin20
Running under: macOS Sequoia 15.4.1

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib 
LAPACK: /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.12.1

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

time zone: Atlantic/Reykjavik
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] igraph_2.1.4.9041     igraphdata_1.0.1.9013

loaded via a namespace (and not attached):
 [1] compiler_4.5.0    magrittr_2.0.3    cli_3.6.5         ragg_1.4.0       
 [5] tools_4.5.0       rstudioapi_0.17.1 pkgload_1.4.0     textshaping_1.0.0
 [9] lifecycle_1.0.4   systemfonts_1.2.2 pkgconfig_2.0.3   rlang_1.1.6