I'm noticing some interesting behavior in `match.call` in some corner-ish cases
that arise when you try to use `match.call` to match a "grandparent" function
and there are dots involved:
fun0 <- function(a, ...) fun1(...)
fun1 <- function(b, ...) fun2()
fun2 <- function()
match.call(
fun1, sys.call(sys.parent()), expand.dots=FALSE,
envir=parent.frame(2)
)
fun0(1, 2, 3)
## fun1(b = 2, ... = pairlist(3))
fun0((1), (2), (3)) # the parens make the substituted args language
## fun1(b = ..1, ... = pairlist(..2))
When the values in dots that need to be substituted are language, they get
substituted by `..n` where `n` is the position in dots. When they are scalars
they are substituted with the scalar. It appears this is done in
'R-3.4.0:src/main/unique.c@1319' in `subDots`:
while (TYPEOF(t) == PROMSXP)
t = PREXPR(t);
if( isSymbol(t) || isLanguage(t) )
SETCAR(b, installDDVAL(i));
else
SETCAR(b, t);
I'm not sure why it is necessary to use `installDDVAL`, which creates the `..n`
symbols, instead of duplicating the language object and attaching that to the
matched call. Certainly from a user perspective I would prefer to see:
## fun1(b = (2), ... = pairlist((3)))
instead of:
## fun1(b = ..1, ... = pairlist(..2))
Do others agree? Is there a structural reason why this cannot be done? Note
this behavior has been this way since at least R3.0 and probably earlier.
Best regards,
B.
PS: sessionInfo():
R version 3.4.0 (2017-04-21)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Sierra 10.12.3
Matrix products: default
BLAS:
/Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRblas.0.dylib
LAPACK:
/Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRlapack.dylib
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] compiler_3.4.0
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