`rlang::as_label()` can't handle calls to `{{`

[NikKrieger]

rlang::as_label(quote({{x}}))
#> Error in switch(type, call = "prefix", control = , delim = , subset = "special", : EXPR must be a length 1 vector

sessionInfo()
#> R version 4.3.3 (2024-02-29)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Ubuntu 24.04.2 LTS
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.12.0 
#> LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.12.0
#> 
#> locale:
#>  [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8       
#>  [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8   
#>  [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C          
#> [10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   
#> 
#> time zone: America/New_York
#> tzcode source: system (glibc)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> loaded via a namespace (and not attached):
#>  [1] digest_0.6.37     fastmap_1.2.0     xfun_0.52         glue_1.8.0       
#>  [5] knitr_1.50        htmltools_0.5.8.1 rmarkdown_2.29    lifecycle_1.0.4  
#>  [9] cli_3.6.5         reprex_2.1.1      withr_3.0.2       compiler_4.3.3   
#> [13] rstudioapi_0.17.1 tools_4.3.3       evaluate_1.0.5    yaml_2.3.10      
#> [17] rlang_1.1.6       fs_1.6.6

^{Created on 2025-10-01 with reprex v2.1.1}

I believe the offending function is rlang:::call_print_fine_type(), which does not account for the possibility of {{. It returns the result of a switch() statement that does not anticipate {{ and has no "default". Therefore, rlang:::call_print_fine_type() returns NULL, which is passed to another switch() within the calling function rlang::call_print_type(). This is where the error occurs, as seen above.

[NikKrieger]

I noticed that this issue only applies when {{ contains a single symbol. If there is more than one argument or if the single argument is a call or a syntactic literal, rlang::as_label() will yield { ... }.

rlang::as_label(quote({{NULL}}))
#> [1] "{ ... }"
rlang::as_label(quote({{1L}}))
#> [1] "{ ... }"
rlang::as_label(quote({{"a"}}))
#> [1] "{ ... }"
rlang::as_label(quote({{f()}}))
#> [1] "{ ... }"
rlang::as_label(quote({{TRUE}}))
#> [1] "{ ... }"
rlang::as_label(quote({{NA}}))
#> [1] "{ ... }"
rlang::as_label(quote({{x; x}}))
#> [1] "{ ... }"
rlang::as_label(quote({{x}}))
#> Error in switch(type, call = "prefix", control = , delim = , subset = "special", : EXPR must be a length 1 vector

^{Created on 2025-10-01 with reprex v2.1.1}

I hypothesize that the C code behind rlang:::call_parse_type() inspects the content of doubled curly brackets and will only yield "{{" if it contains a single symbol and "{" otherwise. Because in all the other cases that don't result in an error, that internal function returns "{":

rlang:::call_parse_type(quote({{NULL}}))
#> [1] "{"
rlang:::call_parse_type(quote({{1L}}))
#> [1] "{"
rlang:::call_parse_type(quote({{"a"}}))
#> [1] "{"
rlang:::call_parse_type(quote({{f()}}))
#> [1] "{"
rlang:::call_parse_type(quote({{TRUE}}))
#> [1] "{"
rlang:::call_parse_type(quote({{NA}}))
#> [1] "{"
rlang:::call_parse_type(quote({{x; x}}))
#> [1] "{"
rlang:::call_parse_type(quote({{x}}))
#> [1] "{{"

^{Created on 2025-10-01 with reprex v2.1.1}

[lionel-]

Thanks for the investigation! If you see a path for a fix, and would like to contribute one, you're very welcome :)
Otherwise we'll get to it at the next round of rlang work.