[Haskell-cafe] Problem on existential type.

[Miguel Mitrofanov]

Your data type GridWidget doesn't have a parameter, yet you use it  
like it has one.

> data GridWidget = forall widget. (WidgetClass widget) => GridWidget  
> widget
                 ^
                 |
NB:-------------+

> liftGW :: (GridWidget widget) -> (widget -> t) -> t