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