Joins involving functions

I have a function that returns a row from a table, e.g.:

create table foo (a integer);

create function f_foo (integer) returns foo language 'sql'
as $foo$ select * from foo where a = $1 limit 1
$foo$

create table bar (b integer);

select * from bar
cross join f_foo(bar.b) as foo;

Unfortunately, this last statement returns an error:

NOTICE:  adding missing FROM-clause entry for table "bar"
ERROR:  function expression in FROM may not refer to other relations of
same query level

Is there a method for joining function calls to tables? The only other
approach I can think of is to write another function that returns "setof".

-- 
Jeffery Boes  <><
jeff@endpoint.com