Type classes question

[Iavor Diatchki]

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


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