change to deriving in 6.7 ??

[Isaac Dupree]

Conal Elliott wrote:
> Thanks, Simon.  The manual deriving is easier than I expected, by
> boilerplate delegation of constraints & methods, as below.  You may want to
> give such an example when you document the change.   Cheers,  - Conal
> 
> -- | Pairing of type constructors
> newtype (f :*: g) a = Prod { unProd :: (f a, g a) }
>   -- deriving (Show, Eq, Ord)
> 
> -- Deriving no longer works on types like :*:.  So:
> instance (Show (f a, g a)) => Show ((f :*: g) a) where
>   show (Prod p) = "Prod " ++ show p
> instance (Eq (f a, g a)) => Eq ((f :*: g) a) where
>   Prod p == Prod q = p == q
> instance (Ord (f a, g a)) => Ord ((f :*: g) a) where
>   Prod p < Prod q = p < q

Hmm, the Eq and Ord instances won't have the efficiency of newtype 
deriving of sharing the Eq and Ord dictionaries with the (,) type to 
which the operations are delegated.  In fact I think that Ord definition 
is not correct because it didn't happen to choose one of (<=) and 
(compare), Ord's minimal definition (and even if it did, it's still more 
slightly-runtime-inefficient not to implement all those methods, 
depending on (f a) and (g a)'s Ord instances)

Isaac