Split up `…` arguments and distribute to multiple functions

Question

Using the following function foo() as a simple example, I\'d like to distribute the values given in ... two different functions, if possible.

Answer 1

1. Why does grep error before sum?

   See that sum is a lot more accommodating with its arguments:

   X <- c(1:5, NA, 6:10)
   sum(X, na.rm = TRUE, value = TRUE)
   ## [1] 56
   

   It doesn't failed because it doesn't care about other named arguments, so the value = TRUE simplifies to just TRUE which sums to 1. Incidentally:

   sum(X, na.rm = TRUE)
   ## [1] 55
   

2. How to split ... to different functions?

   One method (that is very prone to error) is to look for the args for the target functions. For instance:

   foo <- function(x, y, ...){
       argnames <- names(list(...))
       sumargs <- intersect(argnames, names(as.list(args(sum))))
       grepargs <- intersect(argnames, names(as.list(args(grep))))
       list(sum = do.call(sum, c(list(x), list(...)[sumargs])),
            grep = do.call(grep, c(list("abc", y), list(...)[grepargs])))
   }
   

   This is prone to error anytime the arguments a function uses are not properly reported by args, such as S3 objects. As an example:

   names(as.list(args(plot)))
   ## [1] "x"   "y"   "..." ""   
   names(as.list(args(plot.default)))
   ##  [1] "x"           "y"           "type"        "xlim"        "ylim"       
   ##  [6] "log"         "main"        "sub"         "xlab"        "ylab"       
   ## [11] "ann"         "axes"        "frame.plot"  "panel.first" "panel.last" 
   ## [16] "asp"         "..."         ""           
   

   In this case, you could substitute the appropriate S3 function. Because of this, I don't have a generalized solution for this (though I don't know that it does or does not exist).