Multiple values and &optional

[Guy Steele <gls@THINK-AQUINAS.ARPA>]

Just for the record, I had an application today where I really wanted to
write something like

	(MULTIPLE-VALUE-BIND (VALUE &OPTIONAL (FOUNDP T))
	    (FUNCALL (MAGIC-HOOK-FUNCTION THINGY) FUNNY-ARG)
	  (WHEN FOUNDP
	    (CACHE-SOME-STUFF THINGY FUNNY-ARG VALUE))
	  (VALUES VALUE FOUNDP))

The context was that I wanted to have a user hook for a GETHASH-like
protocol, where a second return value indicates whether or not a goodie
was actually found.  However, for convenience I wanted to allow the user
hook function to be any function that returns one thing as well, in
which case one assumes that the thing was found.  In short, I wanted the
second return value to default to T, not NIL.

I ended up writing this instead:

	(LET ((STUFF (MULTIPLE-VALUE-LIST
		       (FUNCALL (MAGIC-HOOK-FUNCTION THINGY) FUNNY-ARG))))
	  (WHEN (OR (NULL (CDR STUFF)) (CADR STUFF))
	    (CACHE-SOME-STUFF THINGY FUNNY-ARG (CAR STUFF)))
	  (VALUES-LIST STUFF))

--Guy