[Haskell-cafe] Re: Monad instance for partially applied type constructor?

[Daniel Fischer]

On Wednesday 29 September 2010 23:15:14, DavidA wrote:
> Ryan Ingram <ryani.spam <at> gmail.com> 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?