'select nextval('seq_name');' in a function ?

Hi,

I am new to postgresql. Using PostgreSQL 7.3.2, I did the following.

testdb#CREATE SEQUENCE principal_id increment 1 start 1000 cache 5;

testdb#CREATE TABLE principal (principal_id int not null, name text, constraint 
pk_principal primary key(principal_id));

testdb#CREATE FUNCTION getnext_principal_id(int) returns int as 'select 
nextval('principal_id');' language 'SQL';

This gives ->
ERROR: parser: parse error at or near "principal_id" at character ...

Please help. Basically, I want to write equivalent of the following Oracle 
procedure:

create or replace procedure getnext_principal_id (retVal out 
principal.principal_id%TYPE ) as
begin
   select principal_id.nextval into retValue from dual;
end;

Thanks much,
Pragati
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