[Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?

[Neil Brown]

On 14/04/11 13:00, Chris Dew wrote:
> class Stream a b c d where
>      (->>) :: a ->  (b ->  c) ->  d
>
> instance Stream (IO d) d (IO c) (IO c) where
>      f ->>  g = f>>= g
>
> instance Stream d d (IO c) (IO c) where
>      f ->>  g = g f
>
> instance Stream d d c c where
>      x ->>  y = y $ x
>

I notice that in all your instances, the last two types are the same.  
So do you need the final type parameter?  Could you not make it:

class Stream a b c where
   (->>) :: a -> (b -> c) -> c

I quickly tried this, and it fixes the errors you were getting.  If that 
doesn't hold for all instances you have in mind, then you may want to 
use functional dependencies or type families to specify a relationship 
between the types.

Thanks,

Neil.