@Neil Brown - That did it. It's not the ideal solution, as all ->>
are 'coerced' into being 'IO x' (if the rightmost term is an 'IO x'.
But it'll do for the time being.
Many thanks,
Chris.
On 14 April 2011 13:50, 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.