Function returning setof taking parameters from another table

Hello.

I've got a function which returns set of records:

select * from f_test(123);

param | val1 | val2
---------------------------
123 | 1 | 17
123 | 2 | 18

I'd like to execute it multiple times with parameters from other
query, like (it doesn't work of course):

select * from f_test(x.id)
join x on (x.id in (123,124,125));

in order to get:

param | val1 | val2
---------------------------
123 | 1 | 17
123 | 2 | 18
124 | 4 | 179
125 | 13 | 18
125 | 15 | 12
125 | 14 | 112

So far the only method I can think of is to use union all with
different parametrs, like:

select * from f_test(123)
union all
select * from f_test(124)
union all
select * from f_test(125);

But it is not flexible, I'd like to have parameters stored in another table.

I can also write a pl/pg function, create a string like this with
unions and EXECUTE it. However, it seems ugly to me.

Is there another way?

Regards,
Marcin