cannot create function that uses variable table name

I have a number of tables in my database that use the concept of
“display order”, which is a field that can be used in an order by clause
to dictate what order the results should come out in.
 
I thought I would be crafty and devise a function that would always
return the highest numbered item in the table.  But it doesn’t work.  It
always gives me a parse error at $1.  Here’s the function:

CREATE OR REPLACE FUNCTION get_last_dsply_order(  varchar,            -- tablename  varchar,            -- id_col_name
varchar)           -- where_item  RETURNS integer AS '  DECLARE total_items integer;     tablename ALIAS FOR $1;
id_col_nameALIAS FOR $2;     where_item ALIAS FOR $3;  BEGIN     SELECT INTO total_items count(*) FROM tablename WHERE
id_col_name
= where_item;  RETURN total_items;
END;
' LANGUAGE 'plpgsql';

Here’s some sample data so that you can better see what I’m doing:
Fileid| accountid | filename     | dsply_order
==============================================    1| account1  | My File      | 1    2| account1  | Another file | 2
3|account1  | YA File      | 3    4| account2  | Hello world  | 1    5| account2  | Hi again     | 2    6| account3  |
Goodbye     | 3    7| account4  | Mom          | 2    8| account4  | Dad          | 1 
=============================================
Therefore you would want to see the last item number used by account2 so
that you can add a new item to the end of the list.  You might do
something like this:
INSERT INTO files (accountid, filename, dsply_order) VALUES
(‘account2’,’Testing’,get_last_dsply_order(‘files’,’accountid’,’account2
’));
 
BTW, it will have a complementary trigger assigned to each table that
upon delete will shift all the items up 1 to fill in the gap left by the
deleted item.  Therefore the count() of the items in the table should
also match the highest numbered item.

--
Matthew Nuzum
www.bearfruit.org
cobalt@bearfruit.org