constrained datatype

[Wang Meng]

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-
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