[racket-dev] Proposal for a "no-argument"

On 07/01/2012 09:27 AM, Eli Barzilay wrote:
> There rare cases where it is useful to have a value that means that no
> argument was passed to a function.  In many of these cases there is a
> plain value that is used as that mark, with the most idiomatic one
> being #f, but sometimes others are used.  IMO, while such uses of #f
> are idiomatic, they're a hack where an argument's domain is extended
> only to mark "no argument".

I believe this is why CL lambda lists allow another variable name when 
specifying optional parameters.

http://www.lispworks.com/documentation/HyperSpec/Body/03_dab.htm
http://www.gigamonkeys.com/book/functions.html

(defun f (&optional (arg 'default-value arg-supplied?))
   (if arg-supplied?
       'real-value
       'default-value))


Matlab's nargin and nargout are another interesting approach.
http://www.mathworks.com/help/techdoc/ref/nargin.html
http://www.mathworks.com/help/techdoc/ref/nargout.html


A (supplied? arg) special form might also work.

- Daniel