ordered by join? ranked aggregate? how to?

What I need is to join 2 tables

CREATE TABLE master(
id INT4
);

CREATE TABLE slave (
master_id INT4,
rank INT4,
value TEXT);

What I need is to make the query:

SELECT m.id, array_agg(s.value) AS my_problematic_array
FROM master AS m LEFT JOIN slave AS s ON (m.id = s.master_id)
GROUP BY m.id;

return the 'my_problematic_array' in order specified by slave.rank

As you probably can guest I don't have any idea know how to do it :/