How to do lazy-evaluation of "..." function arguments in R?

Question

I want to pass arguments to a function that aren't to be evaluated until they're used within the function. As a trivial example, how would I go about if I wanted to pass ... = foo, 'bar' to the following function, while foo is defined within the function:

  myfunc <- function(X, ...) {    
    for (foo in seq_along(X)) {
      cat(..., '\n')
    }
  }
  myfunc(X = 1:5, foo, 'bar')

I tried using cat(substitute(...), '\n'), which seems to merely omit foo.

Thanks!

Answer 1

1) Use eval(substitute(...))

myfunc <- function(X, ...) {    
  for (foo in seq_along(X)) {
    eval(substitute(cat(..., "\n")))
  }
}
myfunc(X = 1:5, foo, 'bar')

giving:

1 bar 
2 bar 
3 bar 
4 bar 
5 bar 

2) defmacro Another approach is to create a macro using defmacro in gtools:

library(gtools)

myfunc2 <- defmacro(X, DOTS, expr = {
  for (foo in seq_along(X)) {
    cat(..., "\n")
  }
})
myfunc2(X = 1:5, foo, 'bar')