Calling a macro from a macro with a variable number of arguments

Question

Say there is a defmacro defined like this:

(defmacro do-something (foo &optional bar)
  (...))

And then there is another one that calls it:

(defmacro something-else (wat baz &optional bar)
  (let ((foo (concat wat baz)))
    (do-something ...)))

What is the best practice for calling do-something from within something-else with the let variable foo, and bar only if it exists as an argument passed into something-else?

I know you could conditionally check the presence of bar like so:

    (if bar
        (do-something foo bar)
      (do-something foo))

Ideally I'm looking for some pattern where I can prepare a list of argument variables and call do-something once, with something like (apply) for macros, where a list variable can be turned into a variable series of consecutive arguments. Or maybe I'm just thinking about this in the wrong manner?

Answer 1

Based on the code you've written, you can simply replace this:

(if bar
    (do-something foo bar)
  (do-something foo))

With this:

(do-something foo bar)

Because optional arguments default to nil. From the Emacs Lisp manual:

If actual arguments for the optional and rest variables are omitted, then they always default to nil. There is no way for the function to distinguish between an explicit argument of nil and an omitted argument.

http://www.gnu.org/software/emacs/manual/html_node/elisp/Argument-List.html