Re: [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
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