Functions with Null Arguments?

Is there anyway way to get the following to work?

table foo
id | name1 | name2
---+---------+------
0 | Abe | NULL
1 | William | Bill

create function prefname(text, text)
returns text as'
declare
name1 alias for $1;
name2 alias for $2;
begin
if name2 isnull
then
return name1;
else
return name2;
end if;
end;'
language 'plpgsql';

if I do
select id, name1, name2, prefname(name1, name2) as pref from foo;

I wind up with

id | name1 | name2 | prefname
---+---------+-------+-----------
0 | Abe | NULL | NULL
1 | William | Bill | Bill

instead of

id | name1 | name2 | prefname
---+---------+-------+-----------
0 | Abe | NULL | Abe
1 | William | Bill | Bill

I assume this has to do with Postgres not executing the function is one of
the arguments is missing...

-
- Thomas Swan
- Graduate Student - Computer Science
- The University of Mississippi
-
- "People can be categorized into two fundamental
- groups, those that divide people into two groups
- and those that don't."