On Wednesday 29 September 2010 23:15:14, DavidA wrote:
Ryan Ingram
writes: Haskell doesn't have true type functions; what you are really saying is
instance Monad (\v -> Vect k (Monomial v))
Yes, that is exactly what I am trying to say. And since I'm not allowed to say it like that, I was trying to say it using a type synonym parameterised over v instead. It appears that GHC won't let you use partially applied type synonyms as type constructors for instance declarations. Is this simply because the GHC developers didn't think anyone would want to, or is there some theoretical reason why it's hard, or a bad idea?
I think there was a theoretical reason why that isn't allowed (making type inference undecidable? I don't remember, I don't recall ...). For your problem, maybe data Vect k m b = Vect [(k, m b)] instance Monad (Vect k Monomial) where ... is an option?