[Haskell-cafe] Can subclass override its super-class' default implementation of a function?

[david48]

On Mon, Apr 27, 2009 at 11:00 PM, siki <gabor at karamaan.com> wrote:
>
> I'm not sure if this is possible at all. I'd like to do something like this:
>
> class A a where
>    foo :: a -> Double
>
>    foo a = 5.0
>
>
> class (A a) => B a where
>    foo a = 7.0

I probably don't understand the question properly, but I don't see why
you would need another class here.

Why not :

module Main where

class A a where
   foo :: a -> Double
   foo _ = 5.0

data Blah = Blah
data Bar = Bar

instance A Blah
instance A Bar where
   foo _ = 7.0

blah = Blah
bar = Bar

main = do
  print $ foo blah -- prints 5.0
  print $ foo bar  -- prints 7.0