[Haskell-cafe] Re: Problem on existential type.
jwlato at gmail.com
Sat Sep 5 07:24:16 EDT 2009
Also, the two definitions for liftGW are exactly equivalent. You have:
liftGW (GridWidget label) f = f label
liftGW (GridWidget textView) f = f textView
The only difference between the two is name to which the data
parameter to GridWidget is bound, which doesn't change the meaning at
all. You should properly have
liftGW (GridWidget widget) f = f widget
There's no way to determine if that widget is a label or textView,
which is the whole point of existential types.
> From: Miguel Mitrofanov <miguelimo38 at yandex.ru>
> Subject: Re: [Haskell-cafe] Problem on existential type.
> To: Magicloud Magiclouds <magicloud.magiclouds at gmail.com>
> Your data type GridWidget doesn't have a parameter, yet you use it
> like it has one.
>> data GridWidget = forall widget. (WidgetClass widget) => GridWidget
>> liftGW :: (GridWidget widget) -> (widget -> t) -> t
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