drvfail007.hs:4:38: error:
    No instance for (Eq (Int -> Int))
      arising from the first field of ‘Foo’ (type ‘Int -> Int’)
      (maybe you haven't applied a function to enough arguments?)
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Eq Foo)