On Tue, 26 Jul 2011, Tim Cowlishaw wrote:
For instance, for a typeclass representing the interface that any
Order type should implement:
class Order o where
price :: o -> Int
size :: o -> Int
I'd like to be able to specify an Eq instance for all types of class
Order in a manner similar to this:
instance (Order o) => Eq o where
o1 == o2 = (price o1 == price o2) && (size o1 == size o2)
You may define once:
orderEq :: Order o => o -> o -> Bool
orderEq o1 o2 = (price o1 == price o2) && (size o1 == size o2)
and then define instances like
instance Order A where ...
instance Eq A where (==) = orderEq
instance Order B where ...
instance Eq B where (==) = orderEq
I don't think there is an easier and still predictable way of defining the
Eq instances.
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