drvfail004.hs:8:12:
    No instance for (Eq (Foo a b))
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      add an instance declaration for (Eq (Foo a b))
      or use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
    When deriving the instance for (Ord (Foo a b))