[Rd] Strange behaviour of do.call()

[Duncan Murdoch]

The knitr::kable() function does some internal setup, including 
determining the target format, and then calls an internal function using

   do.call(paste("kable", format, sep = "_"), list(x = x,
         caption = caption, escape = escape, ...))

I was interested in setting the `vlign` argument to knitr:::kable_latex, 
using this code:

   knitr::kable(head(mtcars), format="latex", align = "c", vlign="")

If I debug knitr::kable, I can see that `vlign = ""` is part of 
list(...).  However, if I debug knitr:::kable_latex, I get weird results:

   > debug(knitr:::kable_latex)
   > knitr::kable(head(mtcars), format="latex", align = "c", vlign="")
   debugging in: kable_latex(x = c("Mazda RX4", "Mazda RX4 Wag", "Datsun 
710",
   "Hornet 4 Drive", "Hornet Sportabout", "Valiant", "21.0", "21.0",
   "22.8", "21.4", "18.7", "18.1", "6", "6", "4", "6", "8", "6",
   "160", "160", "108", "258", "360", "225", "110", "110", "93",
   "110", "175", "105", "3.90", "3.90", "3.85", "3.08", "3.15",
   "2.76", "2.620", "2.875", "2.320", "3.215", "3.440", "3.460",
   "16.46", "17.02", "18.61", "19.44", "17.02", "20.22", "0", "0",
   "1", "1", "0", "1", "1", "1", "1", "0", "0", "0", "4", "4", "4",
   "3", "3", "3", "4", "4", "1", "1", "2", "1"), caption = NULL,
       escape = TRUE, vlign = "")
debug: {

   [rest of function display omitted]

I see here that vlign = "" is being shown as an argument.  However, when 
I print vlign, sometimes I get "object not found", and somethings I get

   Browse[2]> vline
   debug: [1] "|"

(which is what the default value would be).  In the latter case, I also see

   Browse[2]> list(...)
   $vlign
   [1] ""

i.e. vlign remains part of the ... list, it wasn't bound to the argument 
named vlign.

I can't spot anything particularly strange in the way knitr is handling 
this; can anyone else?  My sessionInfo() is below.

Duncan Murdoch

 > sessionInfo()
R version 4.3.1 (2023-06-16)
Platform: x86_64-apple-darwin20 (64-bit)
Running under: macOS Monterey 12.6.9

Matrix products: default
BLAS: 
/Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/lib/libRblas.0.dylib 

LAPACK: 
/Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/lib/libRlapack.dylib; 
  LAPACK version 3.11.0

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

time zone: America/Toronto
tzcode source: internal

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

loaded via a namespace (and not attached):
[1] compiler_4.3.1 tools_4.3.1    knitr_1.44     xfun_0.40