function to operate on same fields, different records?

is this kind of thing possible---?

select gpa(student) from student where id=7121;
select gpa(course) from course where id=29931;
select gpa(prof) from prof where id=1321;

i've got several tables each of which have

create table <various-and-sundry> (
...
a int4,
b int4,
c int4,
d int4,
f int4,
...
);

since i'd like to AVOID this nonsense--

create view courseGPA as
select *,
(a * 4 + b * 3 + c * 2 + d)
/
(a + b + c + d + f) as gpa
from course;

create view profGPA as
select *,
(a * 4 + b * 3 + c * 2 + d)
/
(a + b + c + d + f) as gpa
from prof;

create view studentGPA as
select *,
(a * 4 + b * 3 + c * 2 + d)
/
(a + b + c + d + f) as gpa
from student;

i'd rather be able to do this--

create function gpa( unknowntableTuple ) returns float8 as '
select
($1.a * 4 + $1.b * 3 + $1.c * 2 + $1.d)
/
($1.a + $1.b + $1.c + $1.d + $1.f)
' language 'sql';

any chance of working something like that? if so, how? if not,
well, waaah!

--
does a brain cell think?

will(at)serensoft(dot)com
http://sourceforge.net/projects/newbiedoc -- we need your brain!
http://www.dontUthink.com/ -- your brain needs us!