R: How to use list elements like arguments in ellipsis?

Question

I'm not sure if I labelled my question correct, but I give it a shot: I want to use a package with a function which uses an ellipsis, `func(...)`. All my arguments of class `My_Class` are in a list. As I have quite a lot of arguments, I'd like to avoid `func(arg1, arg2, arg3)`. So ideally I'd like to do `func( my_list )`. The problem is that the function looks like this ``` function(...) { cl <- match.call(expand.dots = FALSE) names <- lapply(cl[[2]],as.character) ev <- parent.frame() classes <- unlist(lapply(names,function(name){class(get(name,envir=ev))})) if( !all( classes == "My_Class" ) ) { stop("an argument to ... is not a My_Class") } } ``` So passing a list doesn't work. I also tried `func(my_list[[1]], my_list[[2]])` and `func(as.expression(my_list[[1]]))`, but even they do not work. As `func` treats the arguments as characters, `func(my_list[[1]])` becomes `chr [1:3] "[[" "my_list" "1"` inside, and `get(name,envir=ev)` crashes. I also tried `do.call(func, my_list)` but this…

Accepted Answer

This works, but I'm mystified by the design choices in that function:

```
lst <- list(
  a=structure(1:3, class="My_Class"), 
  b=structure(letters[1:3], class="My_Class")
)
env <- list2env(lst)
call <- as.call(append(list(fun), names(lst)))
eval(call, env)
```

To trick the function into working, you have to evaluate it in an environment where your list items are objects (this is what `list2env` does, creates that environment).

The terrible thing about that function is that it looks up objects in `parent.frame` based on their names, assuming that the `...` values are indeed names. I really don't understand why that function doesn't just do something like:

```
if(!all(vapply(list(...), class, "") == "My_Class")) stop(...)
```

R: How to use list elements like arguments in ellipsis?

Solution

Problem