IDL already contains syntax for passing arbitrary (and perhaps unknown beforehand) keywords to a routine using the _EXTRA and _REF_EXTRA keywords, depending on whether you are passing a variable back to the caller through the keyword. For example, you can do something like this:

pro my_wrapper_routine, _extra=e
  my_routine, _extra=e

Now, MY_WRAPPER_ROUTINE passes all the keywords that MY_ROUTINE accepts along to it (ignoring any others, use _STRICT_EXTRA on the MY_ROUTINE call to generate an error if an unknown keyword is passed in).

It would be useful to have a similar mechanism for positional parameters, making it possible to write routines in IDL which accept an arbitrary number of keywords or pass along parameters to another routine. For example, passing all parameters after the first two to another routine and printing the first “extra” parameter with the new library routine GET_PARAM:

pro my_wrapper_routine, a, b, _extra
  my_routine, _extra
  print, get_param(_extra, 0)

What exactly would the _extra variable be? I’m not sure. Maybe it isn’t even needed in the GET_PARAM call, but it certainly is needed in the MY_ROUTINE call. Python handles this through tuples and the special * notation, maybe IDL can use structures and a bit of new notation?

But I would certainly like to get rid of code like the following:

case n_params() of
  0: _sql_query = ''
  1: _sql_query = sql_query
  2: _sql_query = string(arg1, format='(%"' + sql_query + '")')
  3: _sql_query = string(arg1, arg2, format='(%"' + sql_query + '")')
  4: _sql_query = string(arg1, arg2, arg3, format='(%"' + sql_query + '")')