[Haskell-beginners] How to avoid repeating a type restriction from a data constructor

[gs]

Daniel Fischer <daniel.is.fischer <at> googlemail.com> writes:

> Use a GADT,
> 
> {-# LANGUAGE GADTs #-}
> 
> data Source x y where
>     Source :: Variable v => { bindings :: v [Binding a], var :: v a }
>                      -> Source v a

I tried this, but every place that I remove the restriction Variable v =>
from something using Source, I get an error No instance for (Variable v) ...