Function returning comma sepparated values (istead of crosstab)

Hi.

Would you please give me a workbale sample of the function which can simulate crosstab (pivot).

I need to send two parameters (group_id, type_id) to this function from the view and this function should return the list of comma sepparated values of that type_id.

So if there are three records with type_id values 'a', 'b', 'c' I need to convert them in one text field like 'a, b, c' from the view (query).

Thanks.