Define a function that accepts query result as a parameter

Hi all,

I have two tables A and B, I defined a function f() on A. The problem is
to process the rows in A whose id has appeared in Table B. To save
time,  I want to to run function f() on only records that appeared in B.
I tried this:

define f(integer[]) as
$$
process records where id in $1
$$ language sql

then I tried to call the function using f(select array_agg(id) from B).
It does not work.
Do you guys know how I can achieve this?

Thanks,
Cheng