hello, Eugene Nonko wrote:

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? instance declarations in Haskell'98 require the type that is being made an instance to be of the form "<constructor> <variable>*", (a constructor followd by some type variables). your last declaration is not of this form (it is simply a variable). if you run hugs with -98 flag, this restriction is lifted. however, note that your last instance will overlap with any other instance as a type variable matches anything. hope this helps iavor

-- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================