[Haskell-cafe] partial inheritance

[Maciej Piechotka]

On Tue, 2011-07-19 at 10:43 +0200, Yves Parès wrote:
> > I haven't followed the thread carefully but why does the bird have
> to be a penguin?
> 
> A bird doesn't have to be a penguin :
> 
> instance (Penguin b) => Bird b where
>      fly = -- fly method for penguins
> 
> Says that every Penguin is a Bird.
> But thinking back about it, there is a problem when trying to define
> the method walk, because:
> 
> class Penguin p where
>    walkPenguin :: ....
> 
> instance (Penguin b) => Bird b where
>     fly = .....
>     walk = walkPenguin
> 
> is kind of awful, because walk has to be duplicated. So, not the best
> way to go...

Note to self: don't respond to ml after 2am. I saw instance but read
class.

Regards
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