Type classes question

Hello, I am trying to define class Dual and few instances as follows: === class Dual a where dual :: a -> a instance Dual Bool where dual = not instance (Dual a, Dual b) => Dual (a -> b) where dual f = dual . f . dual instance Dual a => Dual [a] where dual = reverse . map dual instance Num a => Dual a where dual = negate === For some reason Hugs does not lake the last definition saying 'ERROR dual.hs:13 - syntax error in instance head (constructor expected)'. What am I doing wrong? -- Eugene