How to call table returning function with other table

Hi.

I have a function that is defined like (which works, sorry for the
pseudocode)
create type xxx as (id,...);
create function calcuate_xxx (integer) returns xxx as $$ select $1,
(select sum(amount) from bigtable where something) as a, (select
sum(amount) from bigtable where some-other-thing) as b $$ language sql
stable;

What I cannot figure out is, if I have another query that returns a
single column of those id's, how do I get a table back with the function
applied to all ids (hence multiple rows).

So it is something like "select xxx(id) from othertable where otherwhere
= 't'" except that it mangles the columns into an array. I have tried to
move the function-call into the fromlist (as it is usually done) but I
seem unsuccessful in getting the ID's into it.

Thanks in advance

Svenne