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

[gs]

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

> I meant example code using a GADT for Source, and not DatatypeContexts; and 
> some function where you still need the context.

OK, here are some truncated instances/functions:

data Source v a where
   Source :: Variable v => {bindings :: v [Binding a] ,var :: v a} -> Source v a

instance Variable v => Variable (Source v) where
   newVar a = do bindings <- newVar []
                 v <- newVar a
                 return $ Source bindings v

instance Variable v => Bindable (Source v) where
   bind (Source bindings var) extract target apply =
      do let binding = Binding extract target apply
         --update the new binding
         a <- readVar var

data BindingList v a where
   BindingList :: Variable v => {source :: Source v a, list :: v [v a],pos
:: v Int} -> BindingList v a

fromBindingList :: Variable v => BindingList v a -> IO [a]
fromBindingList b = do update b
                       readVar (list b) >>= mapM readVar

instance Variable v => Bindable (BindingList v) where
   bind = bind . source