[Haskell-beginners] Problem with GADTs and Parsec

[Lorenzo]

> In my opinion, this is one of the few places where existential
> wrappers are really what you want.  If I were you, I would do
> something like this:
> 
>   data ExprE where
>     ExprE :: Expr a -> ExprE   -- existentially hides the expression
> type
> 
> Now you can write your parser to return an ExprE, with the type of the
> expression hidden inside. In order to be able to use the resulting
> ExprE values, you will also want a function like
> 
>   withExprE :: ExprE -> (Expr a -> r) -> r
>   withExprE (ExprE expr) k = k expr

Thanks. This is the kind of solution I was looking for.
The problem is, when I try to access the "hidden" Expr type the compiler
complains:

Couldn't match type `a1' with `a'
      `a1' is a rigid type variable bound by
           a pattern with constructor
             ExprE :: forall a. Expr a -> ExprE,
           in an equation for `withExprE'

      `a' is a rigid type variable bound by
          the type signature for withExprE :: ExprE -> (Expr a -> r) ->
r