Triggers and SQL functions

Can or will postgresql support trigger calls to sql defined functions?

When attempting this I get....

create function effect_of_add_prod(bpchar) returns boolean
as
'update developer
set developer_tot_prod = developer_tot_prod + 1
where exists (select developer_id
from developer
where developer_id = $1);
select true as ignore_this;'
language 'sql';
CREATE

create trigger add_product after insert
on product for each row
execute procedure effect_of_add_prod(product_developer);
ERROR: CreateTrigger: function effect_of_add_prod () does not exist

Searching through documentation reveals only mention of calls to C
functions...is there no way to call SQL functions?

-=pierre