Ellipsis expansion into arguments

[dgkf]

The inverse of #48 is expanding into ellipsis.

I think this should use the same syntax as #48

args <- list(sep = "-", collapse = "; ")
paste("Hello", c("World", "Friends"), ..args)

I don't think there are situations where it's ambiguous. ..rest for collecting into arguments will only happen when destructuring or in a function signature, while ..rest for passing arguments only happens in calls. This might introduce some confusion in situations where the ..rest argument in a function signature is re-used as an argument to default value for another parameter:

f <- function(a, b = sum(..rest), ..rest)) {
}

In this case, the ..rest at the end of the function call collects args, while the ..rest in names(..rest) passes those arguments to sum.

[dgkf]

Reopening, still room for discussion here

[sebffischer]

What is the motivation between the "Duplicated names okay, last instance takes priority"?

This seems to be something not unlikely to cause unintended behaviour / bugs.
I would argue that the f(..args, ..more_args) call in the lines below should behave just like f(a = 1, b = 2, c = 3, c = 10), i.e. result in an Error

args <- (a = 1, b = 2, c = 3)
f <- function(a, b, c) {
  a + b + c
}
more_args <- (c = 10)
f(..args, ..more_args) 

[sebffischer]

Let's say I have the function f = function(...) Reduce(sum, list(...)) and I unintentionally call it with duplicated arguments (using the args and more_args definition from the post above): f(..args, ..more_args).
In that case I think it is preferable to return an error instead of silently ignoring the c element in the args vector.