Trigger Documentation

Hi,

I could not find enough examples about the usage of triggers with
PostgreSQL. Searching in the list files, I found out that there are some
examples in contrib/spi/ directory, but this examples scared me a little
because all the triggers are written in C.

What I need to do now is very simple, I want to make a trigger that check
if some text field is null. If it is, I want to record something on
it. What I was trying (unsuccessfully):

CREATE FUNCTION ins_ip() RETURNS OPAQUE AS '
BEGIN
IF NEW.<field> ISNULL THEN
NEW.<field> := "something";
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER tr_ponto_ins BEFORE INSERT OR UPDATE ON ponto
FOR EACH ROW EXECUTE PROCEDURE ins_ip();

Ie tryed to put something beetwen quotes, double-quotes, nothing
worked. Will I have to write an enormous trigger in C?

Regards,

Alvaro Nunes Melo