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
