I am currently writing a function that I want to pass default arguments if a condition is met. If the condition is not met, no argument should be passed. How can I achieve this? 
I tried it with ifelse and NULL like in this minimal example but it did not work:

```
my_function <- function(.data,
                        .variable = ifelse("var1" %in% names(.data), "var1", NULL)){
  ...
}

```

If "var1" is no variable name of .data and I don't pass another argument on .variable, I want to get an error like "argument ".variable" is missing, with no default". My solution works but I get other error messages.