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!

-- 
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will@serensoft.com
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