"ERROR: Argument of WHERE must not be a set function"?

I want to use a function to generate a list of OIDs to be used in a
subselect.

However, I can't figure out what to return from my function that will
properly work in a WHERE clause.

I tried:
CREATE FUNCTION myTest() RETURNS SETOF oid AS 'SELECT id FROM foo;'
LANGUAGE SQL;

But when I try:
SELECT * FROM foo WHERE id in in (myTest());

I get this error message:
"ERROR: Argument of WHERE must not be a set function"

How can I use a function to generate my subselect? (I want to cal my
function just once, and avoid calling it once per row.)

Thanks,

Drew