Oh, I got it: You want to have:
class Bird b where
class Penguin p where
instance (Penguin b) => Bird b where
fly = -- fly method for penguins
2011/7/19 Patrick Browne
On 18/07/2011 19:14, Jerzy Karczmarczuk wrote:
That's why I suggested how you might do that: for some datatypes, say the Emperors, you specify some special flying method (e.g. dummy or bombing), or you don't specify it at all. And the Emperors won't fly.
-- Here is my attempt data Emperor = Emperor data Robin = Robin
class Bird a where fly :: a -> a -> a walk :: a -> a -> a
instance Bird Robin where fly x y = y walk x y = x
instance Bird Emperor where -- No fly method walk x y = y
class (Bird a) => Penguin a where -- I do not think that I can override the walk method in the sub-class -- must be done in instance of Penguin
instance Penguin Emperor where -- How can I override the walk method in the instance Penguin? -- walk x y = x
Regards, Pat
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