SQL function and input variables

I was makeing an SQL function and got an error which on a sintax that
I thouhgt would work:

CREATE OR REPLACE FUNCTION dicInsertarPalabra(p TEXT)
RETURNS INT AS $body$
INSERT INTO public.diccionario (palabra) VALUES (quote_literal(p));
SELECT COALESCE(codigo,0) FROM public.diccionario
WHERE palabra = quote_literal(p);
$body$ LANGUAGE 'SQL';

Changing p for $1 in the body of the function makes it work. But,
can't we label input arguments like how I did here?

--
Martín Marqués
select 'martin.marques' || '@' || 'gmail.com'
DBA, Programador, Administrador