tcfail157.hs:27:10:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: E m a b
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Foo m (a -> ())'

tcfail157.hs:27:10:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: Foo m b
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Foo m (a -> ())'