constrained datatype

Hi All, Anybody knows why the following code does not work?

class Foo n

data Erk n = Foo n => Erk

test.hs:53: All of the type variables in the constraint `Foo n' are already in scope (at least one must be universally quantified here) When checking the existential context of constructor `Erk' In the data type declaration for `Erk' Failed, modules loaded: none. Is there any reason for this error? -W-M- @ @ | \_/