[SQL] How to create an aggregate?

[Ray Aspeitia]

Not sure if this made it through before I subscribed, so...
Hello all,
before I ask, this is what I have done so far:
-- created this new aggregate function
CREATE AGGREGATE groupconcat_array (
    SFUNC = array_append,
    BASETYPE = anyelement,
    STYPE = anyarray,
    initcond = '{}'
);
-- ran this select statement
SELECT array_to_string(groupconcat_array(oa.order_number), '|') ...
output is text : 46952|46953|46954|46955|46949

What I would like to do is just have 1 function that does the same thing like:
SELECT groupjoin('|', field2) FROM mytable GROUP BY field1
with the same output as my current implementation.
I tried doing that with the CREATE AGGREGATE in conjunction with the 
FINALFUNC parameter set to array_to_string, but array_to_string needs 
2 parameters to function. and I do not know the reference name of the 
STYPE variable while it is in the aggregate function to pass to it. I 
also would like to pass the delimiter to the aggregate as a parameter 
and I am not sure if it can handle that.

I know that this is just being picky, but any insight would be 
appreciated. Thanks.

Ray A.
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