create aggregate function 'count_bool( column_name, boolean )'

Hello,
I'd like to create a function called count_bool( column_name, boolean ) in
PostgreSQL.

this function is similar to the count( column_name ) function but will only
count the trues or falses based on the other input parameter boolean. e.g.
if you pass in a 'true', all the trues will be counted but not the falses --
it's the same but opposite if the 'false' was passed in.

I'd like this aggregate function to be developed with the SQL language
instead of 'C' (for maintenance reasons). From the surface, it appears to
be an incredibly simple job only to find that it's difficult.

In some respects, I can't believe this function doesn't exist in SQL

so now I can create the statement
select distict product_id, count_bool(purchased, true),
count_bool(was_selected, true) from some_table group by product_id;

instead of breaking the query into 3 seperate queries

select distict product_id from some_table;
select count(purchased) from product_id where purchased = true;
select count(was_selected) from some_table where was_selected = true;

Am I missing a detail with SQL based aggregate function development?

Any help would be appreciated.

Thanks!