creating a function with a variable table name

Hey all,

I'm trying to create a function in which the table a query is run on is variable, but I
guess this is not as easy as I thought.

BEGIN
dp=> CREATE FUNCTION stats_addr_dst(date,text)
dp-> RETURNS setof addr_count
dp-> AS 'SELECT ip,sum(dst_packets)
dp'>     FROM(
dp'>       (SELECT dst_ip AS ip,sum(src_packets) AS dst_packets
dp'>       FROM $2
dp'>       WHERE interval=$1
dp'>       GROUP BY dst_ip)
dp'>     UNION ALL
dp'>     (SELECT src_ip AS ip,sum(dst_packets) AS dst_packets
dp'>       FROM $2
dp'>       WHERE interval=$1
dp'>       GROUP BY src_ip) )
dp'>     AS topk
dp'>     GROUP BY topk.ip
dp'>     HAVING sum(dst_packets)>0
dp'>     ORDER BY sum(dst_packets) DESC;'
dp-> LANGUAGE SQL;
ERROR:  syntax error at or near "$2" at character 179
LINE 6:       FROM $2
                    ^
How can I pass the table name?

Thanks!
George